Spell scaling questions, open to both RAW and non-RAW answers

[Fitz10019]

A. The invisibility spell can be cast on an object up to 100lb / caster level. Suppose a caster targets a stone wall in a castle. Would you say the spell fails entirely, because the overall object is too heavy? Or, that 100lb/caster level's worth of the wall becomes invisible? Or just that brick the caster touched?

B. Should a caster of a spell be able to dial the effect down to suit his needs? For example, should a caster be able to make just a spy-hole's worth of one brick invisible (just 10 ounces of material), so he can peek into a room (hopefully unnoticed)? Should he be able to use Summon Swarm to call just one rat?

C. Should a caster be able to choose a shorter time as the duration of one of his spells when he casts, eliminating the need to spend an action to dismiss it? Should he be able to choose that Invisibility or Silence ends 1 minute from casting, instead of the full duration?

[Troacctid]

A. The invisibility spell can be cast on an object up to 100lb / caster level. Suppose a caster targets a stone wall in a castle. Would you say the spell fails entirely, because the overall object is too heavy? Or, that 100lb/caster level's worth of the wall becomes invisible? Or just that brick the caster touched?
Larger objects may be composed of smaller objects. This is determined at the DM's discretion.


B. Should a caster of a spell be able to dial the effect down to suit his needs? For example, should a caster be able to make just a spy-hole's worth of one brick invisible (just 10 ounces of material), so he can peek into a room (hopefully unnoticed)?
There is an "up to" there, so it should be fine, assuming the brick is considered a single object.


Should he be able to use Summon Swarm to call just one rat?
No, that's outside the scope of the spell.


C. Should a caster be able to choose a shorter time as the duration of one of his spells when he casts, eliminating the need to spend an action to dismiss it? Should he be able to choose that Invisibility or Silence ends 1 minute from casting, instead of the full duration?
You can cast a spell at a lower caster level if you so choose, provided it's still high enough to cast the spell. In the case of Invisibility, your caster could cast it at CL 3 (for a wizard) or 4 (for a sorcerer or bard) and have it last 3 or 4 minutes respectively; however, he could not cast it at CL 1, because that's not high enough to cast the spell.