A) it is a 32bit application but can run just fine on windows 10 64 bit.
B) all SCPI commands are based on the instrument and not Htbasic so they should still work. We rum equipment that is 30+ years old on modern day windows 10 machines. The original code was from old hp9000 / hp300 series systems.
C)you can if you want.
In addition I would try to use the utilities that are included with keysight to see if you can talk to the instruments. If you can successfully talk to them from that then you just need to ensure the hpibs driver is being loaded when htbasic loads either from autost or manually.