Spoilers. Avoid if you haven't seen it and want to.
It's be trivial to resurrect *all* of the characters who died in that period. They could say that their personalities were uploaded, and that when Missy was zapped, she was uploaded (which she was counting on), and the same could be said for Osgood and Danny.
Now on a practical level, what you have is a gigantic database connected to a virtual reality, so that the personalities can act and agree to have emotions deleted (why? why not delete them without consent? That only makes sense from a magical perspective).
OK, at the end the place was 'crashing' - but it's entirely possible that that just meant that the VR was crashing and the database was intact, with the characters in their 'last saved format'.
UNIT finds that database, can't really understand it, and manages to extract however many characters you want .. and once they have them, it's possible to keep them going in a virtual reality (have them appear as holograms ) or find a body to put them into.
The cyberman body bit made no sense at all. There's no reason I can think of for the original bodies to matter. IF personalities can be uploaded (I think they can't) THEN the source does not need to be maintained.
When you scan a novel into a pdf file, does the pdf file need the novel to be preserved to continue to exist?
Only if you have 'magical thinking'. In a pure fantasy novel, fine, it's all necromancy, of course you need a corpse, but Dr Who could at least use elements of science fiction that are really, *really* old, and that have addressed this very problem.
See Kiln People as an example, or the books of "The Culture". I think Arthur C. Clarke was writing about this idea in the 1960s, but I'd have to check.
Now, i do not think personalities can be uploaded because in going from analog to digital, you have losses. As such, at best, you have someone who is sort of similar, which is not 'you' any more than a brother or a daughter is.
But Dr Who can make up the rules as it likes, and if digital copying is allowed, but only with extreme limits, then why don't the Time Lords use it themselves? Why bother regenerating? You just upload, get a body grown as in the BSG reboot, download, and even have many copies.
So you'd need a reason within the story to stop them doing it. I think the sampling idea would be the logical one except that it would interfere with the flow of this story.
I have to say I hated this story except for parts - but loved the characters. Plotwise, I felt very disconnected; the sillier parts kept throwing me out of it. I was also disappointed because it would have been interesting if Missy had deliberately set up the moral conflict to include "you can allow a wipe of the database" - and kill these people you love - or you can allow it to continue but it will require my freedom or self-destruct". Use it as a hostage.
When I figured out Missy was the Master (and let's face it, most people had figured out long ago), I did remember Kate Orman asking (maybe 27 years ago) why the Doctor couldn't regenerate as a female. So I'm wondering if that's on the books. I have to admit I'll disconnect at that point, but who knows? Perhaps it will then become even more popular. We'll see.