defending against the chinese has never been a problem since the 70s (dont listen to our media).
the indian side is fully mountainous. the odds needed for the attacker in mountain warfare are about 8:1. so even if we have trouble with infrastructure, the actual defense will be formidable.
The problem is in the offensive. Like the saying goes, the best defense is a good offense. The Tibetan side of the chinese border is relatively flat making it easy (compared to the previos case) for an indian invasion, that is of course if we manage to get supplies across the mountains (Aerial supply for large divisions is not viable). For this, infrastructure is needed on our side which can be difficult since it is mountainous. Then there is the formidable chinese industrial and manpower might which needs a dedicated thread to talk about.
the blitzkrieg pulled off in 1971 was deemed impossible by the western world, but we did it. i think it stands as a testament to indian nature. "we wont attack you, but if you corner us..." so you see, Although the losses are going to be high, at least in theory, I think we can actually capture tibet. But for obvious reasons, we wont.