[TF-Viewer]

Alrighty, some suggestions for the snake & mouse:

For the "fullfilling the sexual aspect" I assume you mean her "getting herself off" during. I can see her managing this in several ways. One being she realizes she's going to lose her arms and hands and be unable to touch herself soon so she frantically begins to masturbate with her hands before the transformation reaches that point. She may or may not be successful in "beating the clock" on that one, if not she could grind her crotch against something on the ground, the carpet, her own clothes, or even the mouse girl if the TFs are done together. She could tell the mouse girl to stimulate her orally perhaps.

If you don't want to fully TF them but still play out the predator and prey scenario, the mouse girl could shrink to the size of a mouse, but remain humanoid in shape and retain intelligence while the (possibly armless naga) snake girl eyes her hungrily.
(though personally, I've always been a big fan of full animal TFs)

Just off the top of my head. Hope it helps.