Who can I MFU?
 

I understand the basic rules for MFU (must book with BA), but it is not clear to me who I can upgrade:

1. Presumably anyone in the same booking as me?
2. Someone travelling with me on a separate booking?
3. Someone not travelling with me (a friend, relative, etc.)?

You can upgrade anyone you like subject to the normal MFU provisos UNLESS you have a HHA in which case it's members only. Just like straight awards.

Correct. However, if there is a change fee (and they charged me 30 quid a ticket when I MFUed my FIL) then the change fee will have to be paid on the same credit card as the ticket was booked under. So I had to get me and FIL on the phone at the same time, which sucked somewhat, particularly as the MFU was a surprise gift for him. I would have infinitely preferred to pay the change fee myself, rather than have him pay it, then me pay him back.

You can dissolve a HHA online, if necessary (as I did). No notice necessary. You just can't recreate it for 6 months afterwards.