1. Disengagement means both sides moving away from each other. so there could be incomplete disengagement even if neither side violated the lac.
As per India, if China has artillery that it moved within a few km of the lac, that means disengagement is incomplete. That is what the problems India is referring to are. So according to India, disengagement is incomplete until China moves its artillery and temporary camps it has behind the lac back to depth areas. This actually began back in June, when China mpved some frontline troops back to this location.
2. As I already explained, India does not claim past the lac in Gogra hot springs. The only disputed area there is that bulge claimed by China. China has artillery guns about 3 km behind the lac near there.
Yes you could be right that the "problems" are infact simply these artillery units so close to the Galwan River. We'll need India's government or China's to release updates on this one to conclude.
Why they don't simply settle on the Galwan river being the border? That seems like it'll make both sides happy enough. I'm not familiar with the claims in this region. Are there mutually accepted maps on China's claims in Galwan/ Depsang, goga hs etc and India's? That were proven reliable and accurate? Would be interesting seeing this since it's not as high profile as Pangong.