which exactly is the most recent clarification for this card because the faq is not clear enough?

What is your problem with the Greatswords? I think the FAQ is the most recent clarification (and the official!).

I think that he wants to be sure that the card trigger its own effect for itself, if it's the case this is right

the greatswords gain one power for each unit enter to their zone, only the first time that they will be played or for every other greatswords unit who enter the zone?

Forced: After a unit enters this zone, The Greatswords gains "1 Power" until the end of the turn.

Every unit (including the Greatswords when it is played) which enters the zone where the Greatswords (if they are already in play) are will trigger the effect. Example: You have the Greatswords in the battlefield. If you play another unit Greatswords in the battlefield then both of them will gain 1 Power. If you play another unit then both Greatswords gain 1 Power.

One trick is to jump with the Pistoliers from one zone to the Greatsword zone back and forth.