Thanks for the advice people.

For your info I've been checking reward availability and what do you know? MIA is completely out now - nada. However, there was MEX availability on 21st December which is great (my partner really wants to see Mexico City) so that will be a nice side trip either before or after Costa Rica. I now need to look at whether Aeromexico or Taca is better to get me to the final destination.

For the redemption, I booked the return leg notionally for the same day and will obviously amend when availability shows for the return leg (ideally on 6th January). However, I have a specific question about this...

Does redemption availability show at 12.01am GMT at T-355? - is it always T-355 or can it vary by a day or so? I'm well aware this is a peak travel time so I need to be checking right on the dot if possible.