1) Soul linkers have a subtype of skills that start with Ka-. These Ka- skills can only be used on the linker him/herself or his/her family members (wife, husband, child). If the linker tries casting this skill on anyone else other than the mentioned persons, he/she will be stunned for a short time and the target will still not receive the buff.
However, if the soul linker receives a Soul Linker Spirit buff, he/she can cast the Ka- skills on anybody without any repercussions.
However (again), a Soul Linker can not cast the Soul Linker Spirit skill on his/herself, so that's where the second soul linker comes into the picture.
2) The best explanation I can think of is to reduce the number of opened clients. So what you do is use the Soul Linker in account 3 and use the Soul Linker Spirit skill on the Soul Linker in account 4, then quickly switch to Sage for buffing purposes. I suppose you're free to just make another account to do the task of the Soul Linker in account 3 if you want.
3) Like I mentioned in 1), marrying the soul linker to a character enables him/her to cast the Ka- buff skills on his/her spouse without the hassle of making a second soul linker in a separate account.
EDIT: Ninja'd by @howrah