A friend of mine's downloaded his copy already. Will I be able to grab the archive file off him and activate it on my account using the serial number I received after purchasing the game?
Unfortunately, no, the archives are user specific. You could log in on his machine and use the download and archive feature in Impulse to get your archive though (right click on the game entry and select "Download and Archive")
Unfortunately I don't have a very stable phone line; it got flooded in a storm not long ago and I still get disconnected every once in a while, probably every half an hour to an hour. Whenever it happens, Impulse stops downloading Elemental and tells me that the file size is smaller than expected. It won't let me resume the download; instead it just starts from 0%. ):
Impulse has the ability to pause and resume a download if the connection drops. If you're getting a smaller than expected error I would guess that the traffic you are receiving is being corrupted in transit. At worst you should be seeing it retrying the download multiple times.