Increasing your kick height requires regular dynamic stretches as part of your warm up and static stretches after exercising.
Simply stretching your groin and hamstring is not enough to produce the flexibility required for high section kicks. You need to stretch all of the lower body muscles (calves, quads, hamstrings, glutes, groin, hip flexors) and joints (ankle, knee) to improve your control over the height and power of your kicks.
It is also important to stretch your lower back, obliques and abdominal muscles. These core muscles are utilized fully in throwing all kicks, but especially roundhouses.
On the leaning back vs. not leaning back debate, I believe that the middle ground is the best option. You definitely don't want to lean too far back so that you are off balance and the slightest counter motion from your opponent will knock you over. However, I also find that if you don't lean back far enough you are left open for either a jab/cross, even when using your lead arm as a counterweight crossing in front of you and having your rear arm guarding your temple.
Essentially, throw the kick while still having a body posture that allows you to remain balanced but keeps your head out of range. But really, a lot of this doesn't matter in the fighting moment, as good technique goes out the window in most fights. That's why you see guys like Badr Hari throwing high roundhouses with both of his arms flailing out to the side and dipping over + leaning back. (and subsequently falling to the canvas).