It sounds like you have found someone who has a good idea of what is going on with you. I think it is always wisest to go with those with the best understanding of your history-there are so many factors. with the pcos, and the surgery, it really would be irresponsible to speculate further. Kinda a chicken or egg thing.....
I will say, Estrogen dominance is one of the key suspects with us, and your problems are very common mirena side effects-(including inability to lose weight). I think he is wise to recommend removal. there are a few recent posts about more natural birth control.
as far as and endo is concerned-don't be surprised if your tests come back "normal". Most of them test for very specific conditions, and the more generalized illnesses we seem to suffer from seems to rarely come up on the medical diagnostic radar.