Yes, I've updated my cMP and also my Retina MacBook Pro to later wireless cards, but they're either a simple swap or there's an adapter available. What I've not been able to find is a newer card with the ribbon connector of the Mini. Sounds like no-one else has either.
AFAIK, no.
Of course you can go for TB -> PCIe enclosure -> PCIe adaptor -> BCM94360 card...
However, the cost may be even more expensive than the Mac mini. I don't think it's worth the cost. It also makes the whole setup very bulky.
So, my own conclusion is "possible but not worth".
However, since BT4.0 is the real limitation, wifi card isn't that important for Watch Unlock. I think go for the USB BT 4.0 dongle route is feasible. However, I never try that myself, and never study the Mac mini forums about that. You may go to there to have a look and see if any relevant discussion.