Couatls and Change Shape

[ATHATH]

Couatls have the following ability:
Change Shape. The couatl magically polymorphs into a humanoid or beast that has a challenge rating equal to or less than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (the couatl's choice).
In a new form, the couatl retains its game statistics and ability to speak, but its AC, movement modes, Strength, Dexterity, and other actions are replaced by those of the new form, and it gains any statistics and capabilities (except class features, legendary actions, and lair actions) that the new form has but that it lacks. If the new form has a bite attack, the couatl can use its bite in that form.
I have some questions about it:
1. Does a couatl's (DEX-based) bite's to-hit bonus and damage change based on its new form's DEX stat (and proficiency bonus?), or will it always have a +8 to-hit bonus and a damage of 1d8+5 (without buffs or the like to modify those values)?
2. If a couatl is in a form that has a Multiattack action that includes a bite attack, can the couatl use its bite instead of the form's bite in that multiattack? What if an ability like Rampage specifies needing to make a bite attack?
3. If a couatl is in a form with an ability that's dependent on a non-STR, non-DEX stat (such as a CON-based breath attack or the like), does that ability get modified to reflect the couatl's relevant stat modifier?
4. If a couatl is in the form of a were-creature and it uses the bite attack of its natural (couatl) form, will that bite force a save vs. contracting lycanthropy?
5. Can a couatl repeatedly make nonlethal attacks with its bite against a 0 HP creature without harming it until it fails its save against the couatl's poison? In my headcanon, it'd basically just be chomping onto the creature's arm and injecting a constant stream of divine poison until it succumbs.
6. If a form that a couatl takes has an ability with an X/Day restriction or a recharge roll restriction, can a couatl bypass that recharge period by just turning into another member of that same species? What if it instead takes the form of a different type of creature of that same species (like, say, a Y mage instead of an ordinary Y warrior)?
7. Which abilities count as "class features, legendary actions, and lair actions"? Does innate spellcasting (as opposed to just the Spellcasting feature) count? Does racial psionic spellcasting (such as those of the gith) count? Does a reaper of bhaal's passive aura of murder count? Does a Hobgoblin Captain's Leadership ability count? Does a Tabaxi Minstrel's Inspire ability count?
8. If a creature's stat block mentions attacks with a manufactured weapon or an armor bonus due to having armor/a shield, does a couatl temporarily gain that equipment while in the form of that creature? What happens if the couatl drops that equipment?
9. Does a couatl gain the weapon, saving throw, and/or skill profiencies of a form that it assumes? For example, if a creature's stat block mentions attacks with a certain kind of weapon, can a couatl in that form wield a magical variant of that weapon with proficiency? If a creature's stat block gives it proficiency in stealth, perception, or history, could a couatl use those proficiencies while in that form?
10. Does a couatl need to have seen an example member of a humanoid or a beast species in order to turn into one, or can it just turn into a generic member of a species or organization that it's merely heard about? Can it turn into a specific person it's seen before in order to impersonate them?
11. Does anyone know of a good resource for searching a creature type for a specific ability instead of just for a specific name? I'd rather not accidentally spoil myself on some adventures while scrolling through a list of CR <=4 Humanoids.
12. Is a couatl's flight magical? Can it fly using its wings in an antimagic field (assuming that it's not a summoned creature, which would just wink out in such a field, IIRC (... would it?))?
13. What are some good Humanoid (or Beast, although I'm already familiar with the utility abilities of most Beast forms) forms to use with the Change Shape ability, whether for utility purposes or for combat? A form with some kind of laser vision or the like would be nice, just so that I can come even closer to completing a summoned couatl's collection of Superman powers.

These questions do actually have a point, as Couatls can be summoned by Conjure Celestial (a PHB spell) and then bound using Planar Binding. In fact, summoning a couatl seems to be the intended usage of Conjure Celestial, as only two CR <=4 Celestials are in the PHB: the CR 4 Couatl and the CR 2 pegasus (which can already be summoned for a much longer duration by Find Greater Steed, a spell of a much lower level than Conjure Celestial).

And yes, I know that the answers to most of these questions will be up to my DM, but I'd like to know the community's/Sage Advice's consensus on them first, if possible.

13. And finally, what was WotC smoking when they decided to make a creature with a bajillion utility spells, passive truesight, bonkers base stats, immunity to nonmagical weapons, and (effectively) LESSER SHAPECHANGE a CR 4 creature? Why didn't they just place it at, say, CR 6-7 (while keeping the shapechanging ability limited to forms of CR <=4) and allow Conjure Celestial to summon Celestials of CR <=6-7?

[MaxWilson]

11. What are some good Humanoid (or Beast, although I'm already familiar with the utility abilities of most Beast forms) forms to use with the Change Shape ability, whether for utility purposes or for combat? A form with some kind of laser vision or the like would be nice, just so that I can come closer to completing a summoned couatl's collection of Superman powers.

It really, really depends on how your DM interprets "except class features." For example, the CR 4 humanoid "Warlock of the Archfey" has at will disguise self, mage armor (self only), silent image, speak with animals, and 1/day Conjure Fey. It doesn't say it's a class feature, but it clearly is intended to mimic PHB warlock invocations (except for Speak Without Animals, which isn't a PHB invocation, but still seems to be thematically a class feature), even though it's listed in the stat block as "innate magic" not "spellcasting." In my game, if a Couatl polymorphs itself to look like a human, it looks like a human, not a human Warlock of the Archfey, and certainly not with a Warlock of the Archfey's powers. But if your DM allows it, knock yourself out. See if you can get access to Hobgoblin Devastator powers too, and Iron Shadow teleportation. Weretiger form is unnecessary because the Couatl is already immune, but Quetzalcoatlus form is pretty good (high damage when dive-bombing, and high mobility including Flyby, and still compatible with Couatl's poisoned bite). In fact the Couatl's native form is already pretty decent in combat because of Constrict as well as the poisoned bite.

Orc Blade of Ilneval is pretty good especially if you have multiple orcs in the party (multiple Couatls or even orc PCs). Have to ask the DM though whether the orc appears armed and armored already as part of the Polymorph, or needs an external source of equipment.

Knight has good damage and comes with a free quasi-Bless that stacks with Bless.

Gityanki Warriors are a hard-hitting choice. Duergar Mind Masters even more so, and they can even help PCs get additional attacks (on a failed DC 12 Int save).

Orc Fang of Shargaas hits hard and has a quasi-Blur effect via Darkness.

Meazel teleportation could be useful.

Inspired hit hard and have some nice spells (like Hex and Hold Person) although again see above about class abilities.

[ATHATH]

... If a couatl takes on the form of a dwarf (or a duergar), can it attune to a dwarf-only magic item, like a Dwarven Thrower? If so, what happens when the couatl returns to its normal form?

[ATHATH]

See if you can get access to Hobgoblin Devastator powers too
Wait, do couatls keep their ability to cast spells while in a Change Shape form, or do spells count as "other actions"?

[MaxWilson]

Wait, do couatls keep their ability to cast spells while in a Change Shape form, or do spells count as "other actions"?

Ask your DM. The MM isn't written very carefully (e.g. Yochlols which turn into mist cannot ever turn back into Yochlols, by RAW, because turning back requires an action and being in mistform incapacitates them, which denies all actions) and there's no real use expecting it to answer corner cases like this question.

[J.C.]

Wait, do couatls keep their ability to cast spells while in a Change Shape form, or do spells count as "other actions"?

Per the Change Shape description . . .

The spells are kept since they are part of the game statistics that are kept.

The Couatl loses the actions on its stat sheet (grouped under Actions) other than the Change Shape action (and the bite action if the new form has a bite action as specified).

[ATHATH]

that the new form has but that it lacks
Since couatls already have the Innate Spellcasting ability, do they not copy variants of the Innate Spellcasting ability from the forms they assume?

[J.C.]

Since couatls already have the Innate Spellcasting ability, do they not copy variants of the Innate Spellcasting ability from the forms they assume?

If the new form's innate spellcasting exactly replicated the Couatl's innate spellcasting then it would not acquire the innate spells as the spells would not be new capabilities. But, no form exactly replicates the Couatl's innate spells so the Couatl simply acquires the new form's innate spells.