I don’t think Sonos does any stacking of discounts. I recall ‘complaints’ before about the issue. 

You cannot get a double discount. If the BF offer is greater than 15%, you should not activate your 15% as you will only get a 15% discount.
I have just ordered 2 x Era300 which have a 20% BF discount.

Once activated, the upgrade credit does not expire.

However, the upgrade credit cannot be used in conjunction with any other discount or Offer Price as per the T&C’s of the upgrade program.