as i said before lelouch is confrimed to be dead, by one of the writers and he is listed as deceased in the offical r2 guide book. but that doesn't they won't revieve him or something. there are many animes that revive their dead characters. or they could make an alternate ending like you said. but as of now lelouch is DEAD.
now as that being the end of code geass. i don't think so. just because lelouch is dead doesn't mean the series is over. sunrise has indicated that there might be more code geass in the years to come. there are still many unanswered questions.