According to what I found online, the yeast will attenuate anywhere from 73% to 77%. So if your original gravity was 1.050, then I would estimate the final gravity to be 1.010. The advice you received to pitch dry was fine. There are may opinions on pitching dry yeast right over the wort versus not, but I've done both and found absolutely no difference.
Pitching at 78F is not bad. The yeast appreciates temps 62F to 72F though so you were slightly high. You did get it down to 65-66F which was good but that first 24 hours was tough for the yeast and it was likely stressed. Also, once you pitch your yeast there is no need to stir it, it'll do it's thing.
You didn't mention aeration so I am assuming you didn't really do that before you pitched the yeast. No biggie. Well it is, but technically right now it is not. It's a low gravity beer so the yeast would have been pretty okay I think. That 78F though was tough.
Airlock activity is never a sign that fermentation is done, which I think you probably know.
What I think happened was you stressed your yeast at 78F then you shocked it moving down to 66F; if that happened fast. What I would have done was very lightly swirled the bucket/carboy one time, just rotate it on an end. Don't splash the wort, just make it move and get some of that yeast that dropped back up there.
You waited a good amount of time but if you're sitting at 1.018 and your ABV is off by that much, I would have started googling stuck fermentation. It is not uncommon for extract batches to stop at 1.020 but not when they start at 1.050, not in my experience. Also, the fact that increasing your temp caused movement from 1.021 to 1.018, I would have waited another couple days after getting the 1.018 reading to get one more.
In truth, it may have never moved. I'll go back and say that the yeast could have died a slow death and what didn't were working but slowly. Racking to a bottling bucket roused the yeast, you heated it up more and that woke them up and they wanted to eat that sugar.
If you can chill to 70F going forward, your yeast will have a better chance to survive. Unless you're doing a saison or something that appreciates warm fermentation.
It does suck that you lost beer to this but the fact that only one bottle blew and it was plastic is luck.