Can the lowest hp target change multiple times during a hero's turn? For example, if Tempest uses chain lightening to hit a target that then becomes the lowest hp target, then does that target become immune to damage?
Yep, highest and lowest can change multiple times in the middle of an action. It's assessed constantly. Tempest could theoretically get the benefit from Gene-Bound Shackles against EVERY target when using Squall, for instance, if they're close together in hp and he applies damage in the right order. Haka can do the same with Rampage and Punish the Weak, and so on.
Confirming this is correct
Thanks to both Foote and JimmytheRat.