While playing a character who joined the Legion, I've noticed the Imperial helmet seems to be bugged and doesn't really count as Heavy Armor (says it's Heavy Armor), nor does it take advantage of the Heavy Armor perks, so I thought fine, I'll go on a hunt for that unusual closed helm that hardly anyone can seem to find, as it turns out it's in the east empire warehouse under Solitude, but as far as I know you can only get in there during the Thieves Guild quest.

Is there another of those Closed Imperial Helms in the game somewhere? or a way to gain Access to the Warehouse without breaking character?
(Not a thief)

There may be a back entrance, but it won't be marked on your map. Northeast of Solitude there should be a cave along the coastline you can enter...

I think that it might have something to do with the quest Missing in Action, because there is supposed to be a method to free the prisoner if you are wearing a legion uniform, but the uniform in question isn't in the game.

i got one from Meridias temple , right at the start just as you enter theres a dead imperial on the floor , the helmets next to him.