Is it possible that virus latency can occur with ebola, so that even after 21 days of quarantine, it remains within the infected patient's cells?

So far - no: Mutations might become a new issue as time goes by. But, so far - no. Don't panic.

It would cause sxs: by day #21. Per CDC, incubation period is 2 to 21 days w average of 8 to 10 days. A different topic - is transmission from ill person - According to the World Health Organization – men who have been infected with Ebola Virus can still transmit it through semen for up to seven weeks after they have recovered from active illness. See: http://www.who.int/mediacentre/factsheets/fs103/en/ ***