OK, thanks for the suggestions: mainly, isolate the system drive and see what happens. It's strange that the update from Win 8.1 to 10 went ok, but the big updates since then have been a pain. Seems almost worth it to keep putting off 1703 and wait for the Fall one. In any case, I mentioned not including the setupact.log because its last time mark was most of 20 monutes before I started the last try.
You can also try doing a clean boot first- and no AV or 3rd party firewall. This only helps if there's a conflict with a running program- which I once experienced. It doesn't affect drivers, for example.
I've tried finding something relevant based on your log, and whilst I can find matches to parts, nothing that looks directly helpful. You can try that yourself of course.
The only thing that stood out to me was the error code for the software distribution folder, which that link you mentioned said can get in the way of a successful update. I don't know if it's really a problem here, but I deleted it. Thanks, good ideas to try.
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