What is the difference between 穿 and 穿上?
穿 and 穿上 are usually like English 'wear' and 'put on'.  The sentence 我正在穿上毛衣 means 'I am putting on a/the/my sweater'; I think this is quite unambiguous.  The sentence 我正在穿毛衣 is a little bit odd (to me), but can be interpreted as 'I am wearing a sweater'. 

穿上 is an action, which has a beginning, middle, and end. 穿 is often a state, if you like, with no beginning or end.  Not always of course: for example, in imperative sentences like 趕快穿衣服! "Hurry up and get dressed!" the 'hurry up' makes the non-stative meaning clear. You don't HAVE to use 穿上 to mean 'put on' then, but it helps make the action/state distinction explicit.  

脫 'to remove (clothing)' is unambiguously an action to begin with, so it doesn't need a resultative ending to disambiguate it.  脫下 is possible of course; to me it emphasizes both completion (clothes no longer on me) and a more detailed process. One example: I might use 脫 for removing shoes, 脫下 for a suit (jacket and pants).