You have 2 activities per night included, you can do 4 activities in one day if you wish. Most people will do the falconry, camel ride, desert drive and dune bashing.
AFAIK they allow all guests to stay as long as they want in the spa area. As you’re checking in late they will probably let you eat dinner on your last night. That said I wouldn’t worry to much they are extremely accommodating.
Whoops - GVA - I actually meant if we're only staying *1* night (so two activities total). But I guess it would be the same?