Do I always gain triple experience while I am linked to my recruit?

No, only when you are partied with a character on the linked account and both players are present when slaying the monster or completing the quest. Also, if the two characters are not the same level, only the lower-level character is given the experience boost.
First method, your main accounts could get 4 mounts.

You do have to purchase the game in order to remove the restrictions of a trial account. You can't refer a friend on a trial account.