Yeah I agree with you about having lost weight perhaps a bit fast. My story is damn similar (age, how long I was overweight for, weight and such) except that I've lost about 25 kilo a couple of years ago. I've pretty much kept it off since, fluctuating a few kg here and there. My skin has tightened up -a lot-, I think because it took a while for my skin to adjust to my body's new size.
Having said all that though, I haven't lost the extra 10ish you have, so I don't know what difference that would make to my skin.
BUT, it's a good example that skin can bounce back a little over time (probably also because we're young). Staying well and truly hydrated through-out weight loss and afterwards is essential though.
As far as stretch-marks go, I still have mine and always will, but they have faded away to my skin-tone. I'm going to find out this summer whether or not having a tan will make them stand out, or if I'll be too happy with how the tan hides all the other flaws that I won't care too much
If you want to try something to accelerate the fading, then apparently using vitimin E creams daily is helpful. I've done this but think the fading was more due to time and staying a lighter weight.
Anyways! Goodluck with all of this.