Let me start by saying I understand your frustration. Cutting back on my words that got overly long, the main problem could very well be the worn hooks on the body that hold the magazine. I would try solving the problem here first, then elsewhere. Worn hooks can also be the reason for the focus issues as the position of the magazine can be off.

It shouldn't be expensive to get a new pair of hooks machined and installed if you can't find the original Hasselblad part.

I would suggest a focus bracketing test, with the magazine pressed firm against the body (with the help of tape or strong rubber bands for example) and without. Putting a few little USAF focus charts behind each other just a little apart and focusing on the middle one, then evaluating the results will tell a lot. (You can do it with any set of small objects, resolution charts just help making the test quantitative.) Often we remember what we focused on when taking the shot, then just notice during printing the main subject is not critically sharp and forget to find the best focus just a little front or behind it.

It is often necessary to shim focusing screens. Their alignment is quite often off.

I wouldn't give up on that beautiful camera yet.