QIf I give this follower in my EX area +2 attack with its second activated ability, then summon it, does it keep its +2 attack?

AYes. When an effect is applied to a card in the EX area, that effect will persist even if the card is played or summoned.

QCan this follower's second activated ability only be played while it's in my EX area?

AYes, that's correct.

QI have 4 cards in my EX area. If I play this follower's first activated ability, which card do I put into my EX area?

AYou would put this follower into your EX area, as the cost must be paid first. Since your EX area would then be full, the top card of your deck would remain where it is.

QCan this follower's first activated ability only be played while in hand?

AYes, that's correct.