I don't care what the spread sheet says or the calculations it will be Mistake to go with the unconventional Z axis you will be creating a tuning fork and this will show in the finished cut guaranteed.!!

Regards the belts then problem with one long belt arranged like this is you can't get enough tension on it with out applying pull on the screws and it becomes unwieldy belt length because you have to make sure the motor pulley as enough teeth engaged and this often means longer belt.
So I'd be more inclined to have 2 short belts going from screws to rotating idler shafts so can get nice tension on screws then longer belt to same shafts on another set of pulleys that you can get good tension on the belt using motor as tensioner. (like doodle in pic) More expense and time but will be better than one long belt.
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To be honest it's far easier and stronger for machine to run a brace across back then just one belt can be used. In practice you'll more than likely find you never put any thing on bed that it gets in way off and you can still pass long boards thru.
It would also mean you can lift the ballscrews back up closer to gantry which will stiffen it up slightly and make easier to build.