1) Meh 76-78 is pretty high and the act of fermentation tends to raise temperatures even more. You might get a lot of esters and phenols but if you let it sit in primary for a few more weeks it should clean up just fine.
2) A fast fermentation isn't uncommon... with a lot of my beers the majority of fermentation takes place within 48 hours of the first signs of fermentation.
3) 64*F is an awesome temperature for most ale yeasts. But since fermentation is probably already done, you should let it warm back up so the yeast can clean up after themselves. Remember, temperature control is only paramount during the height of fermentation (i.e. probably no more than 5 days after pitching.)
4) It's possible it's due to the temperature but it's too late now. Like I said before, best bet is to let the yeast clean up for a couple weeks and it should be fine.
But the airlock is not an indicator of fermentation or the completion thereof. It is simply a tool that allows CO2 created during fermentation to be released while not allowing outside air to get in. Essentially, your airlock is a lying jerk that probably licks all of your silverware while you're not home.
But in all seriousness, fermentation can still be going on well after the airlock stops bubbling. Also, there are a lot of posts on here along the lines of "my beer started fermenting again because the airlock started bubbling again" when in actuality it's because the beer warmed up and some CO2 was released because of it (gases dissolve into solution much better at colder temperatures.)
I hope that helps