Is it okay to modify or work on game's github repository after the submission date? how it evaluate, from the repository that had been made before the deadline?
I'd guess as long as the source is available, that's all that matters. To be extra safe, you could always use another branch until voting ends. If for some reason they needed to verify, the last commit you made before submitting has a hash with date/time etc. so that could be matched to your itch version.