OK, here's my thoughts (based on experience)
The BMFA A and B tests are tricky with multi rotors - if your drone has any form of stabilisation/flying aid that you can't switch off (such as the Phantom's altitude hold) then you can't take the A or the B test and have to take the BPC which is in every way identical to the A test but specifically for aircraft with flight aids - The CAA will accept the BPC as an alternative to the A cert.
If you want an A certificate specifically then you need to fly something without atti mode. As you correctly said BMFA Heli certs are acceptable but of you took them a long time ago (like me) the BMFA will have (almost certainly) 'lost' your records - unless you still fly with the same club or have your original 'pass' document then you'll need to do the tests again
If you want a B certificate, you need to fly a drone with full manual control.
As you have found out, some clubs are quad friendly, some aren't but not many clubs will welcome you with open arms if all you want to do is use them to take your test.
I wasn't aware of the BMFA extended insurance but as I intend to fly commercially it wouldn't help me. My policy is with Coverdrone and cost just over £400 for a year/£34 a month - whatever policy you take out has to meet EC785/2004.
You'll also have to send in an operations manual - that's no easy task to put together.
OK, conjecture bit now but based on chats with the CAA and BMFA instructors.
To fly within 150m of congested areas etc requires extended permissions and these are granted on a case by case basis - just because you have a PFCO doesn't give you an automatic right. One of the things they consider is safety/redundancy so getting this permission with a quad won't be easy.
Good luck with your application - mine's currently in the pipeline with the CAA