How to safely delete .inProgress Time Machine backup?
The next backup interval prunes and cleans this folder. If you have 24 hours of backups and still have a hung inProgress, I would open a case directly with Apple.
With hard links, it’s unlikely you are using any space on these files, since another interval already covers them or the next interval will reuse that space.

The risk of messing with the backups is you ruin that destination so unless you are willing to make that destination read-only and start backing up to a new destination, I would not mess with this manually - even to try to convince tmutil to delete that non-interval.
I have had great success using time machine utility to delete whole backups and letting the system run to pre-clean and post-clean these progress folders. When the progress folder remains for more than a day of successful backups, you usually have another problem and poking at a system with one failure isn’t likely to address the main problem. You could also try to bypass and clean up a large file if that helps you get space back if deleting intervals doesn’t prune enough space.