i've trained quite a bit in the past, in the rare TKD school that had lots of hard sparring, and still do plenty of heavybag work as part of my exercise regimen. Rest assured, I can generate a helluva lot of power landing roundhouse kisk with my instep, and guys I've trained with and trained under can deliver far more. Bear in mind, T&B, the bones of your foot comprise the two main load-bearing members of your entire body and can (and do) absorb a punishing amount of force doing any number of thing, such as running on pavement. Bone is one of the hardest, most dense substances in existance.
Conversely, if the shin was so tough, we'd not have any gag-inducing videos showing kickboxers snapping their shins.
I've connected with people's elbows lots of times, and while it does hurt like a mofo hitting them with the instep, I've not known anyone to break any bones.
Also, TKD kicking done properly involves turning your hips over as you say; examine stills and video of good TKD kickers and you'll see what I mean. The mechanics of TKD and Muay Thai kicking are not completely different.
I'm getting ready to get back into training now that I have time, money, and medical insurance. I've settled in a school that teaches BJJ and Vale Tudo, the latter includes boxing and Muay Thai. I'll kick how I'm trained, and will indeed be interested in what I find doing so.