Size questions on Overrun and Sentinel

[Creyzi4j]

Some questions about overrun and sentinel. Coz It seems a bit unrealistic if applied in game.

A gnome fighter vs a tarrasque. The tarrasque moves away and provokes an attack of oppprtunity. If the gnome hits, does the tarrasque's movespeed become 0? Sounds a bit weird having a small creature stop a gargantuan's movement.

If the tarasque tries to use overrun action (DMG pp.272) on a gnome, and the gnome gets a critical 20 and beats the tarrasque's roll. Does the tarrasque stay in place and not overrun the gnome?
It is pretty unrralistic considering their differences in size.

Extra question: if you are swallowed and you cast a fireball spell inside the creature? Does the creature need to roll a DEX save?
How much dmg would the creature take?.do they take extra or not?

[Unoriginal]

If the tarasque tries to use overrun action (DMG pp.272) on a gnome, and the gnome gets a critical 20 and beats the tarrasque's roll.

There is no such thing as a critical 20 for skills, in D&D 5e. And the Tarasque would have at minimum advantage to its roll, meaning that the Gnome would have to roll more than a creature who has +10 and who do the test twice.

Or you can just rule out that the size difference is too great and the gnome cannot even try.

[PloxBox]

seeing as this is a fantasy game, I'd allow it. To the former: the tarrasque felt the pain from the hit, and is very confused as it looks around for what hit it. It finally notices the small creature, letting out a bellowing roar before unleashing a flurry of attacks against it.

Edit: for the latter, I'd rule that if the gnome succeeds then they would instead go Dragon's Dogma and cling to it, able to climb it as difficult terrain (1/4 walk speed seems appropriate, right?).