If "Susan was already gone" were true, would a vampire really have laid down on the alter to willingly die as a weapon against all the other vampires? That was pretty clearly still Susan making the choice.
I doubt Uriel could or would have told Harry what happened to her afterward, though. Not being allowed to know for sure seems like another one of the Rules.
She did make a choice. Remember? So did Harry. He knew full well or figured that when he
revealed Martin's treachery, she'd go postal and kill him.. It was still her choice to go postal knowing the consequences if she did kill. When she laid herself down the change wasn't quite
complete, but was when Harry cut her throat.. If it hadn't been the reversal of the generation
spell wouldn't have worked. So she wasn't human when she died, now an exception might have
been made for her. It could be that she works for Uriel or something like that, but he wouldn't tell
Harry because there are other plans for him, plans that are beyond even Uriel's pay grade.