2. While Geoffrey Bayldon does give an spot-on impersonation of Hartnell in the audios, I don't know how well it would work live. I can give a spot-on impersonation of Nixon or Reagan, but I certainly don't look like either one.
Bayldon wasn't impersonating Hartnell at all. The Unbound audios are "alternate universe" Doctor Who stories. Bayldon was one of the first choices to play the First Doctor before Hartnell was hired, so his stories are a "what if he'd said yes in 1963" scenario.
As for David Troughton, hasn't he recorded some of the Target audio books featuring the Second Doctor? I think it should be easy to tell from that whether he'd pass muster in an audio sense as his dad. I wouldn't support either David or Sean or anyone else impersonating any of the earlier Doctors on screen. I didn't care for Hurndall being brought in either. They barely got away with it back in 1963, and only because they broke the fourth wall with it by including a clip of Hartnell at the beginning and basically saying to the audience "work with us one this".
Jon Culshaw, the Dead Ringers guy, would work well as a stand in for Tom Baker, but there's no point now that Tom Baker himself is doing audios again (and when he's no longer available, I won't want to see or hear from the Fourth Doctor again under any other actor - I don't want anyone impersonating Lis Sladen either). An exception is the Companion Chronicles, in which we get to hear Katy Manning, Nicola Bryant, Mary Tamm and others impersonate their Doctors, but those are special cases since it's usually the companion telling a story. And besides, hearing Nicola Bryant impersonating Peter Davison is actually kind of hot.
I understand it's sad for fans of a particular Doctor to know there won't be any more First, Second or Third Doctor adventures featuring the original actor and let's enjoy the Fourth while we have him. But once the actor is no longer available, then if we want more adventures, we need to rely on original novels. There are tons of books out there featuring all 11 Doctors, and there's every sign BBC Books might be warming up to doing more books featuring the "classic series" Doctors, given the print-on-demand reprints that were supposed to start circulating this summer and the launch of a new series of Target novelisation reprints. I think for the 50th BBC Books is going to do something with all 11.
Alex