well they did reply, and my customer support tech was LOVELY really, and i did a whole bunch of things like cache clearing and something with the command screen... but none of it helped when i restarted the computer and tried again.
what DID help was a suggestion on the official forums to cancel my pre-order (which never did charge my card, it seems) and just buy it now. and now it's installing with about 8 minutes left. so there we go!
just posting in case this helps anyone else later. happy simming y'all~