How to 'thin' out limoncello?

Yeah, water. More water, some more sugar, and maybe toss some lemon slices in there too.

Also, if you're willing, leave it open for a day or two, let some of the alcohol just evaporate. I wouldn't, because I'm a cheapskate and think if I can dilute it then I waste nothing, but seems logical that if you can let alcohol evaporate off of tinctures for better taste, you can do the same thing for beverages. What's limoncello other than lemon peel tincture and sugar?

My suggestion for next time would be to use a very high proof vodka rather than Everclear, go with something you're willing to drink in large quantities. I use Everclear to make extracts for flavorings and stuff, but I don't actually drink the filth.

It shouldn't freeze.

The problem is that you are going to dilute the flavor of the lemons if you just add water, simple syrup, or even vodka. The best way to dilute it will be to infuse water with lemon rind and add this.

Usual way to cut proof is with spring water. If it were me, I'd take samples and cut it with different brands of spring water till I got the taste I was looking for. A LITTLE more simple syrup might be in order too.

Oh, I'd taste the water by itself to start. I think one of the best is Evian, as it has NO taste. But, to each his own.