I made an account and never got my email?

That took a long time for me, too. Nearly 1 1/2 hour.

It has been 2 days and no response. So I emailed support. No response. Not even an automated response.

Thats a bummer, dude... Sorry for you!

there is no "multiplayer" per se, as far as I understand the Race The Sun site. You get to share your score with 3 other people (that would be the "coop-relay racing" mode) but there is no actual interaction with the other players.
GOG's description is wrong in this case. There is no "multiplayer, coop" ... only "share your score and check back later to see how much other players added to it".
In the same vein, there is no real point in buying the game more than once to get more keys. You still won't be able to race with your friends.
It IS a good casual single player game though.

(btw. I wrote that in a review, and apparently it has been saved, as I can't write a new one for Race the Sun, but it doesn't appear. wazzup gog?)

Isn't there a coop "relay" mode for 4 players? (i.e. you race until you crash, then send your save to another player who gets to race from that point, etc...)

yes. supposedly there is that "multiplayer" mode, although, to be honest, when I think "multiplayer" I think interaction. By this standard any game with an online scoreboard has "multiplayer"