If this is possible with puzzle 66 address, then any address with tx in mempool is possible to be taken. Just the range is a little bigger but still computing the pvk should be the same easy as is for puzzle 66.
Then all BTC is in danger. You may want to try steal any other tx in mempool when there is pubkey available.
No.
Properly generated bitcoin private key is insanely hard to crack (it's 256 bit). Whole bitcoin strength is based on a fact that cracking 256bit is generally considered as impossible.
This can change when quantum machines will be introduced but currently you will not crack it even having PK available.