I dont know the real history, but further to what u mention, Guru Nanak did undertake 4 major journeys called ‘udasis’ and one of these was to the east, during which he is believed to have visited this part of sikkim including Lachen and Guru Dongmar Lake. He is known as ‘Nanak Lama’ to the local Buddhists. At Lachen Monastery they have a record of his visit.
I was really surprised when a local buddhist in Lachen came to me and through sign language communicated to me that how their ‘Nanak Lama’ and our ‘Guru Nanak’ are the same. For me it is an indescribable experience to meet a stranger in a strange land and be greeted with familiarity and shared respect of what till then to me was ‘our’ Guru.

Yes the Gurudwara there is built by the Army, (and i guess they run it as a ‘Sarva Dharma Sthal’ ).. which is amazing. That would explain the newer building and pictures etc.

I know this doesnt prove Guru Nanak’s visit to this amazing place, however i think it is enough for you to maybe tone down ur skepticism.