1) Yes. The bearings in a stepper motor are not designed for axial loads. And you'll have backlash, as the stepper shaft will have some axial play in it.
2) Good enough is subjective. I would say no, but you may find that it works for you. Two bearings per rail distributes the loads better, and adds a lot of rigidity,
3) not really.
4) Depends how fast you want to go. The 350oz of torque is when they are not spinning. The faster they spin, the less torque they have. At some point, they'll stall, or lose steps. Stay below that speed, and you'll be fine. What that speed is would depend on the drives, power supply voltage, and acceleration rate.
5) My preference would be to mount the rails on the sides, with the screws below them, getting everything off the table, for easier loading, unloading, and general use.