Although check-in starts at 7:45 pm, you don't HAVE to check in then. If you were doing the terrace viewing and needed to snag a good seat, yes 7:45 pm (and even lining up before is a good idea), but with the Plaza Garden viewing, it really isn't necessary. My DS9 and I did the Plaza Garden viewing party in December and we were at least 30 minutes late (got caught in a rain storm at BTMRR) and I'm actually glad we were as it doesn't take us an hour to eat dessert! We ate our dessert and then were rather bored as we waited to be escorted to the garden. Another plus of being late (though I can't guarantee this will happen to you). They ran out of seating on the overly, bright and crowded upper terrace and seated us on the "mezzanine level" (right above the terrace viewing guests). We were the only ones on that level, so it was quiet and a little more ambient. I'm hoping we get that seating again when we go in August.
I would have your guide end your tour at Casey's, grab a bite and then try to get to Tomorrowland Terrace by 8:15 - 8:20 pm. This will give you plenty of time to have some desserts and then head over to the garden to see the show.