Is the ability of coronavirus to penetrate the central nervous system also the cause of seizures and convulsions observed in late-stage patients?

This is very much the case. The reports and the videos you have seen of patients convulsing are real. This is something that will occur in a significant percentage of people with a severe infection who are quite seriously ill and have the underlying vulnerabilities, in most cases due to advanced age and/or preexisting health conditions that leave them in a weakened and more vulnerable state to acquiring a serious infection when exposed to a pathogen.