I think you misunderstood what the OP is saying. The OP is saying they intend to go into a foundation course anyway, as they meet the criteria, and therefor ABB will meet the minimum.
The OP is also saying the extra A level is just something to do during their gap year, not as a resit.
To the OP:
A) Yes, if you cash in (the school does this) your A2 grades, those are the ones your offer will be for. This is the case as most med schools only look at your grades you achieved in the first two year sitting of your alevels. For you, this is your ABB. Also, most med schools don't accept re-sitters, especially the ones you listed, so they most likely wont look at the A level you did in your gap year.
B) Your offer will be unconditional as you already have cashed in A level grades that meet the minimum requirement, and there is "no condition" for you to meet. Just remember you still have to sit the UKCAT and meet the minimum threashold and also you need to do well at interview!
Good luck OP! Please email all the med schools in advanced saying these are my grades, I want to take a gap year, but in that gap year I want to do another a level for fun, and I dont want this to count towards meeting the conditions for my offer, if I was to get one. That way, if there are any problems, you have a record!
Enjoy your gap year!