Yes you understood that whole thing right, but when rem said her life was extended she wasn't implying that misa had been alive for a long time. Just that because she was supposed to die young she's now gonna live much longer and then add on whatever years she get from gelous. I do wonder though how that works fate wise.
Speaking of fate I always appreciate how in all media, life and fate are one of those things that are forbidden to mess with since it messes with the natural order of things. Even in a supernatural world there has to be consequences.