There are patches elsewhere in the mailing list to allow one to mount
a broken-due-to-transid-mismatch  filesystem readonly, which should
work in this case.

Regarding your backup regimen, consider using rsync instead of dd:
after the initial backup, rsync can update the existing backup _much_
more quickly, making it practical to do a backup every night, or even
multiple times a day.  dd also has the downside of potentially
_really_ confusing btrfs if it ever sees the backup and the original
at the same time.