Thanks Ramesh,
here's the second Fixlog. I noticed after this that Windows booted considerably faster than it has been doing since the failed update; it had been on a "please wait..." screen for a minute or two before starting Windows, this now doesn't happen.
SFC can now get a little further; it now says
Beginning system scan. This process will take some time.
Beginning verification phase of system scan.
Verification 6% complete.
Windows Resource Protection could not perform the requested operation.
DISM can also get further; it says
Deployment Image Servicing and Management tool
Version: 10.0.22621.2792
Image Version: 10.0.22631.5335
[===========================62.3%==== ]
This doesn't seem to get any further after leaving it for a while.
Certain applications still fail with the COMCTL32.dll error or the Side-by-side configuration error.
Just to clarify something I mentioned in the original post, in order to boot into Windows, I had to change the Boot option in msconfig from "Windows Rollback (\windows)" to "Windows 11 (C:\WINDOWS)". (I feel like the way I mentioned this originally was a bit ambiguous and it could have some relevance.)
In case it helps (now that I was able to run SFC and DISM somewhat), here's updated CBS logs: CBS 20250608 15.43.zip
Thanks