We at Centova are not involved with Nullsoft's licensing procedures so all I can do is repeat what I've heard, but I believe DrO commented on a prior thread here that while it's probably technically possible to use the same key on more than one stream, you're legally supposed to have a separate key for each stream.
As for company/reseller keys, I'm not aware of such an option but maybe DrO can comment if he notices this thread. This has been asked on a couple of other threads though so you might want to search in case he replies to one of the others.