So I had a large lag spike while playing and I was wondering two things. Does it count as input buffering as I had more time to react and if not, is a laggy video sufficient to submit?

It most likely depends on how fast the run is. If it is a top run in a major category, they may reject it, but otherwise I don't think that the mods will hold a laggy game against you.

tangent is more than likely correct here. as long as the run isn't under like 2:30 you should be okay but don't quote me on that