I figured that its easier to do one big post with all my questions.
depending on were you are in the game these could be spoilers

First off i have progressed far enough were where i have found Sky Haven Temple and made it a base for the two Blades (the chick from Riverwood and the Old guy from Riften) And because Lydia is my current companion i made her a blade. My question is can i quit using her and go get the archer elf from riverwood and make him a Blade as well, im hoping i can use Sky Haven Temple as a companion holding area of sorts, or does once i quit useing a companion do they go back to the spots that you found them.

Second question- i have that marriage necklace, is Lydia a companion you can marry and if so how do you start that?

Im on the last part of the Forbidden Legend quest: I have found two parts of that necklace and the last one is in the Saarthal ruins, but when i went to the location the only door in was locked and it said i need the excavation key. Where do i find the key.

I got a side quest from the greybeards to go find the word of power in Sliverdrift layer and i cleared most of the ruins that i could, the only way to progress is through i locked gate just inside the main chamber. Has anyone else made it through and if so how did u do it.

And finally i found the Alftand ruins that i cleared all they way through and at the very bottom of the ruins after you battle the giant centurion there was a podium that said "unlock the dwarven mechanism" or somthing about activating it, but i couldnt cause i didnt have a key. did anybody else do that and if so what do u need to activate it.

1-????
2-You cant marry Lydia
3-you can get into the saarthal ruins by doing the 2nd mission of the college of winterhold
4-????
5-????