I really like sliding glass doors as opposed to swinging doors for ease of feeding; I can open the door just enough to get a rat on tongs inside, and still have the glass between my hand / body and the snake. But substrate can get stuck in the ruts, so it's a toss up.

Depends on how much the dividers are. You really don't need a divider, though.

If you've got a good thermostat (Johnsons, Ranco, Herpstat, Helix, etc.), yes flexwatt will get basking temps where they need to be. Ambients can be more tricky, especially for arboreal snakes. I've got a space heater in my snake room, I keep the room at 70 and the UTH's in the cage are enough to bump ambients 5 or 10 degrees. The space heater also gives me peace of mind if any of my snakes get out, knowing they won't freeze.

If you're still shopping, I highly recommend Animal Plastics cages. I've been using them for all of my larger snakes for years and I absolutely love them.