Hi, it is sure the deadline is finished.
About the floor : you need to revert face on sketchup that's why it is invisible in SL.
About textures : The solution I found for that is to texturise with planner but it is not allways beautiful. As suggested by Chic Aeon, You need to MAP the textures to your model by unwrapping. I heard about but don't know how. You and I must learn to do it on blender.
As explained, the physics must be a build in same sizes as the main one where you put walls or cubes where avatar don't have to walk and ope where avatar can walk
About export/import/export : When finished on sketchup, the best method I found is to export as 3ds format. You import to blender, you select all (A and A) and you join all (Ctrl + J) and you export as Dae. You have to do same for the physic build.
I'm not sure if what I said has sense, but it works with me. I have two big problems : the LI is not so good optimizing and the texture. Working with sketchup is a real pleasure. I dream to find people using sketchup to second life so we can exchange tips.