Can I use a multichannel amp to bi-amp my speakers

You can use a multichannel amp but I wouldn't purchase one just to bi-amp. I noticed more improvement getting larger amps and not bi-amping. An active crossover will also help with bi-amping.

... was considering using preamp/s I have, with a vintage SimAudio Evolution W8 for sale here on Audiogon as a bi-wire (instead of bi-amp), or considering ModWright KWI200 to simplify ( and would bi-wire also). Was lead to believe that bi-amping gave better results regarding control and clarity: true?

To biamp effectively, you really need an active x-over. I don’t know how you could use that while also using a single multi-channel amp to drive the speakers.

I think the amps you are considering would work better than a multichannel amp. I also noticed more improvement in sound with a better single speaker cable than bi-wiring with a lessor cable.