1) Does your party's level affect the rate or type of treasure left by some types of monsters?- don't think so...

2) Is there a way to increase the rate at which monsters drop items?- As far as I know... No. But Sneak Gloves increases the steal rate.

3) Is it even worth it? At least with Malboro Tentacles, you win them every time you kill a Malboro.- In my opinion... no. Why do you want 99 of everything anyway?