I assume you're talking about a separate ADR, not the CP package?
Of course, it depends on the day and how close to Christmas, how crowded the parks are, etc. but I would say that's not enough time without the package to get a seat. And I think if you want the 6:45 show with the package, they will book you dinner a lot earlier than that. We ate at 5:30 with the package for the 8 PM show. Non-package guests line up 2 hours in advance or more, often. With the package, we still had to get in line 45 minutes ahead of time.
As for Illuminations, you will be kind of rushed after the 8 PM show to get a GOOD spot, but probably okay if you're not being picky about where you watch from.