I dont think not allowing comments would affect your account or visibility or ranking. The only thing that matters when youtube shows ur videos in the search results are KEYWORDS and then NO. of Views and nothing else.
I didnt understnd ur 2nd question ''Can I rank more than one video for the same keyword on the same account?"
I think u can add as many videos with the same keywords on the same account. That would even help you to achieve control over the suggested list of videos which pops up beside ur upload
what happened to my fonts???