KilgoreTrout

I think the thumbnail will only show up in windows explorer if you turn on the "preview pane" in explorer.  Also, it won't look very good unless you turn on the large thumbnails.

The only thing I can think of is the "minimum display size" setting in your preferences.  Mine is set to 0 but if you have set yours to some large value you could get this problem.

Just because your plan file is "no bigger" then other plan files does not mean that it will work the same.  It may generate many more surfaces when you create a camera view.  It might also contain something (like a library symbol) that Chief doesn't like.  I would try deleting things to see if the problem goes away which will help you figure it out what is causing it.  If all else fails, post the plan or contact tech support.

If you turn off "hide terrain intersecting building", then your terrain will go through your foundation walls.  This may not be a problem in the plan you are working in, but if you had a basement then it would be a problem.  When I run into these problems, I manually create my own hole in the terrain.  I start with the room polyline for the exterior room, modify the shape as needed, and then convert it into a terrain hole.

I would divide the rooms like it looks like you have done.  Then, in order to get the deck planking to go the directions you want it, you need to trick Chief into treating them as separate deck areas.  I do this by changing the floor height by 1/16".  As long as the floor heights are different then Chief won't join the framing.

Do not turn off "connect island rooms" in your general wall defaults. 
 
The problem is that your utility closet is creating an island room.  An island room is any room that is not connected to any outer walls forming an "island" inside your house.  Island rooms cause all kinds of strange and mysterious problems which is why you don't want them.  Turning off the default will just cause you more problems later.
 
Instead, you should always connect your island rooms by manually drawing your own invisible walls.  Once you have correctly drawn your own invisible walls, you can delete the automatically created one.  If you don't want to see them in your plan views, then just turn off the layer.