Hi,
As one of Dr VdV's former patients who underwent a very radical and generally very successful transformation, first of all I just want to wish you the best of luck. I'd also like to insist that FFS is actually not too bad. Sure there is almost always a lot of post surgical swelling and bruising (really bad in my case), but the worst of it disappears in two to three weeks. Otherwise, there is almost no pain worth mentioning, even during the days immediately after surgery.
Apart from that, I also wanted to say that I had lipo-filling done twice, once in July 2011 and a second "refresher" in Dec 2012 both because a lot of the fat was reabsorbed after the first session and because there was just not enough fat available to do everything Dr VdV wanted to do first time around.
Regarding lipofilling, reabsorbtion is real issue, probably even more so for skinny people. The fat cells might remain where they were injected but you lose the fat they initially contained.
I'm a lot older than you so in my case that might have been a factor. In theory, for someone as young as you, the combined effect of HRT and lipofilling should be pretty good which brings me to a question.
How long have you been on HRT? While you should benefit greatly from chin and jaw surgery no matter what, you would be well advised to have of couple of years of HRT under your belt before considering doing anything to your cheeks. Like Carly said, putting on a little weight probably wouldn't do you any harm either..

Wishing you all the best!
Donna