Fairly certain that it’s the case that each tick checks the resistances of the monster individually, somebody please correct me if I’m wrong.

Given that, in the case where the RR goes away the third tick would do less damage than ticks 1 and 2. It would have the same BASE damage of 2000 but the actually applied total damage value would be lower because the resistance of the target has gone up.

In the other case, yes, more damage will be received in future ticks. Again, the BASE damage will remain the same at 2000, but the decreased resistances will apply to that value to increase the total amount applied.

AFAIK it MUST be checked every tick, because of various chance on attack or chance on crit procs that can alter the damage mid duration. I’d assume that also means RR gets checked.