Regarding the Android SPL, it is not so hard if you read carefully, but I am sure I read it more than 3 times before I understood it ! Try this response here (answered) Install /e/ on FP5 with newer Security Patches - #2 by aibd.
Vendor SPL is something different. Did you keep evidence of the Android SPL on your previous installed (and locked) system [1]. (For all I know it was 5 may 2025, but it is critical.)
True – this date is the date of build, you quote builds one day apart.
No, because anti rollback is triggered by one thing only Android SPL as explained in the link I started with.
The FP5 install page you quote does apply just the same for community or official with the exception
Warning: The bootloader is lockable in official builds only. The procedure to lock the bootloader will not work on community builds.
[1]