Mjoll is my wife, will she attack me if i kill him? Also, why does Mjoll even outside on Lakeview property strut around only in her underwear?

If you Kill Aerin Mjoll will no longer follow you, no clue how it might effect a marriage. Just dont get caught or if you side with the Dawnguard he'll get killed if she ends up kidnapped.

Because you took her default armor and followers who aren't following will only wear there default clothing.

ok ty, anyone know what her default armor is so I can give her back the same type or is the armor specific to her?

Its specific to her so you can't just buy another set, Now default items can only be bought from your spouse while they are following you and your at the house but if all their default items are taken they'll respawn.

In addition to her default armor Mjoll has a default ring. So tell Mjoll you want her to follow you and then ask what she has for sale. (all while at your house). Because she's following you her store inventory will be closed leting you only see her regular inventory you should see an unenchanted ring but it then dismiss her as a follower. She'll later respawn in her default armor.