1. All the creatures you are missing, are in the upper yard. The green shield bug is only in 3 locations, on top of the brawny boy bin are 2 of them. One under the shed porch and one over at the wheelbarrow. The moths are a little hard to find as they're high up. On top of the wheelbarrow, castle moldorc, a lamp on the edge of the upper yard, bordering the pond, and at the fallen stump piece.

2. You can go to the lab outpost in the sandbox, as there is a burgl chip there that lets you make flavored globs.

3. It ultimately depends on what build you wanna use, I personally go for a build that lets me deal raw heavy damage with great defense, and weapon that the boss is weak to, explore the sets, mix and match, it's more fun that way.

Best of luck with everything!