I'm pretty sure you can go to the 5:00 first and eat afterwards. You pre-pay so they don't care which order. I think they used to give your your pass at your meal, and didn't charge you before hand, but now that they do charge you in advance you should be able to follow your plan. One thing though, is I've read that some people felt that it wasn't dark enouhg at the 5:00 show to fully appreciate the candle light part. You might consider eating at just before 5:00, going to the 6:45 show and then seeing Illuminations later.
i'm going for a 4:45 dinner PS. I figure by the time we are seated, we won't be eating until 5:00 or so and should still have plenty of time to get to the 6:45 -- and it will be darker by then.
Hope that helps.