National Electrical Code Articles and Information

1. Yes, if the switch is a unit switch that a marked "off" position, is part of a fixed heater, and disconnects all ungrounded conductors [424.19(C)(3)].

2. Yes, if it meets all four of the conditions listed in 424.20(A)(1) through (4). For example, it must be designed so the circuit cannot be energized automatically after it has manually been put in the "off" position.

3. Yes, and that must be per Article 210 for non-motor operated equipment [424.22(A)].

4. You could, but you would need supplemental overcurrent protection of not more than 60A [424.22(B)]. And those supplemental OCPDs must be factory-installed or factory-supplied and meet other requirements [424.22(C)]. There are also engineering considerations involved; think out the design carefully before deciding to run oversized branch circuits with lower-rated OCPD at the point of utilization.
   
5. Yes, they are. And they can be sized as small as 100% of the nameplate rating of the heater if all three conditions of 242.22(D) are met.