What Jeff said.
I have yet to jam a Hasselblad, and i have been using those for many decades.
Reports of jammed Hasselblads are, without fail, reports of either run down fifth-hand equipment that has never been serviced, or of operator error.
So never buy a camera (that applies to all brands) from someone who is too cheap to have it serviced but rather sells it on when it stops working properly. Have equipment serviced when needed (when that is depends on how nuch t is used). And read the (extremely thin) user manual and remember the little there is too remember.
Then you will never have a problem.
Now your other questions:
- Yes. If a lens fires while off camera, all you need is a coin to cock it again.
- When the body fires without lens attached, you simple recock the boddy. If there was a loaded film magazine on the camera, the frame in the gate will probably be lost, so leave the magazine on when you recock the thing so you wind that exposed frame on and get a 'fresh' one in the gate.
- No. You cannot be too slow in attaching a lens. There is no such possibility of things happening as in your description.
- Yes, you can turn the axle coupling camera to lens from behind, through the rear of the camera. Should you ever need to (which you will not).
But if things happen as in your description, that will not help. Another screw must then be used to disengage the camera's key from the lens. After which it has to be reset and adjusted again: a job for a qualified technician trained to do so.
- The BIG QUESTION:
I have said so above already, but though the internet is full of this nonsense, the chance of running into a problem is about as big as winning the Multi Million Mega Jackpot in a lottery. Though many like to think there is a chance they will wn that jackpot, i wouldn't hold my breath waiting for it. So do not worry!
The "unjamming tool" is a Big Waste of Money.
Not only because you will never need it (peope who do should really stop and think, and find out what they are doing wrong). But mostly because you do not need to spend Big Money on a thing that does what any cheapo small screwdriver does even better.
I.e.: don't fall prey to those who sow fear to reap protection money.