I've tried letro to combat tren related gyno (from not controlling estrogen effectively) with decent success....BUT, once the glands develop, they are there for good!!! The best you can hope for is to halt the progress of the gyno and/or shrink the lump.
Mine isn't even noticeable unless my bodyfat gets super low, or when i raise my left arm (left nip), and even then, I'm told it's only really me who notices. If you've been running the letro for 12 days and notice nothing then that would raise some red flags for me about the quality of your AI, especially at 2.5 mg ED. But, everyone has differing degrees of responsiveness.
Having dealt with flare ups over the years I can only give bro knowledge about my own experiences for you going forward. Adex controls flare ups when on cycle effectively, as does aromasin, but letro is superior to those for keeping it totally in check IMO. Nolva is inferior to AI's, as it can halt the progress but doesn't really seem to shrink the lump at all.
Drawbacks are that letro is more difficult to dose appropriately without getting the shitty sides (loss of libido, screaming joints). Most gyno should limit itself anyways. When you're off, the gland will likely shrink...but of course, you can control that on cycle too. Also, from this point on you'll need to be aware that you will be prone to flare ups, and choose your compounds and doses more appropriately, as well as providing estrogen control.
For now, I think you might as well start tapering it down and begin your nolvadex to recover. Your hormones stabalizing out might provide some relief to your issue. Surgery is the ONLY way to eliminate gyno completely.