[cypher]

Putting the mesh inside or outside is personal choice I think, I've heard some people say inside so they don't chew the plastic & others outside so they don't chew the mesh, I put mine on the outside & not had any problems at all but I'm sure it might depend on the individual ham as well.
I think the more mesh panels you can put in the better not just to allow for good ventilation but it also allows for more easy interaction plus you can attach things to the mesh if you want to. At a minimum I would say a large panel on the top & front of the bin, if you only have one on top you don't really get any air flow.
I can't quite imagine the way you're describing the bin for depth, sorry I need more coffee! Basically though you want to try to make the substrate deep in as much of the cage as possible, just one area doesn't really work so try to aim for at least 15cm all over, more if possible.
Connecting bins safely so they are totally escape proof is not easy, I've not tried it & wouldn't really recommend it tbh.