There is a $25 credit for the first person, but not the second as that fare is considered "free".
The $75 for the transfer was included for us even though we had a one day deviation. We left it as we stayed at a hotel by the airport instead of going to London. We were going to do that anyway as we did not want to drag our bags to London. Note the transfer is only good from the airport even with the deviation. There may be a difference on that if you book one of their hotels.
For our trip back from Reykjavik, DW said we are not staying hours at the airport, so she booked an excursion for us back to the airport. We were told the $75 BOGO fee would come off automatically, but it did not, and I had to go to guest services to get it removed.