1. My garters usually poop within 2-4 days after eating, depending on what they have been eating. Worms go straight through them, mice take a bit longer, fish are somewhere in between.
If she's acting normal, then there's little reason to be concerned. My guess is that the rest of the poo is just getting lost in the substrate.
2. Try to avoid upsetting her.
That's the best advice I can come up with. That, and continued exposure.
3. About 15-20% of her bodyweight per week should be fine.
