QIf I banish a Naterran Great Tree token as part of the cost of this follower's activated ability, can I resolve the Naterran Great Tree's ability before making my choice for this follower's activated ability?

ANo. You must choose 1 before you pay the ability's cost.

As for the Naterran Great Tree's ability, you would resolve it at the next confirmation timing after resolving this follower's activated ability.

QI have 4 cards in my EX area. When a Mount card leaving the field causes this follower's ability to trigger, can I choose whether to put a Dutiful Steed, Bullet Bike, or Arcane Personnel Carrier token into my EX area?

AYes, you can. Similarly, if there are 3 cards in your EX area, you may choose your preferred two.

QIf I have multiple copies of this follower on my field, and a Mount card I control leaves the field, will each copy's ability trigger?

AYes, they will.

QCan I play this follower's activated ability multiple times per turn?

AYes, you can.