Tweeza, one thing to consider with your catch pan is that if the sides are damp you probably also have larvae crawling out of it. They can't climb a vertical surface unless it's damp. That's why it's recommended that you keep sawdust or similar bedding in a collection bucket.
mrgrackle described a good method for collecting the immature, light colored larvae. Here's a few photos that show my sophisticated immature larval collection system:
The collector with fish pellets placed on BSF compost:
After 2 minutes:
11 minutes:
After 40 minutes:
I'm not sure what to recommend for separating the larvae from the compost. I think a good approach might be like what is done with earthworms where you attract them to specific areas with fresh food. As you can see by my photos the larvae will migrate to certain foods very quickly (for larvae that is )
One idea would be to layer some fresh food in a separate container like a plastic storage box. Over the new food place a wire mesh, maybe 1/4 inch, and then empty the active BSF larvae and compost onto the mesh. Most of the larvae should migrate down to the new food pretty quickly. If you had a day or two to wait I think almost all of the larvae would move below the mesh and you would just lift the mesh out. If you had two stackable containers you could cut most of the bottom out of one and attach the mesh to the bottom. Then put the scraps in the unmodified container and then stack the one with the mesh bottom into it, making sure the mesh touches the new food. I would probably put the lid on the top container and let it sit for a day or two, checking on the progress occasionally.
Tweeza, I'll help you even if you don't buy something from me. The information is free. I appreciate your input.
edit: I just saw your second post. I've read that BSF larvae will eat onions and I think they would love some fennel too. It's true the larvae can and will eat almost anything except paper and grass, but the adults don't eat anything. They only live a few days and they live off of stored fat. I've read that they drink nectar from flowers but I can't say much about that. The (dark) prepupal larvae don't eat either. In that stage they only want to find a quiet dry place to pupate.
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