If CUN counts as North America (I think it does) I had no issue to book a seat on an award I redeemed for a non-member (I travel for work on a revenue ticket and take a family member with me).
As always with TK - if you have issues with web site, use App - one of them should work
You were right - couldn't select seats for the 1st segment on the website, but I was able to do so using the app! Thanks for the suggestion.
Meal selection was erroring out on the app, but at least it wasn't greyed out like the website. Will try again later.