Is there any reason why the KB/mouse support on PC's can not be made available on Android tablets?
It should be available, at least with Android. The os supports mouse and keyboard, there is no need to change the game for this. Every Bluetooth device is supported by Android (I had a broken screen once and used my mouse for WhatsApp...)
Similarly, can the Android touch screen support, with its extra icons, be made available on PC touch-screen tablets? Ideally, it would be great to have a single integrated UI that works for both.
It's a bit harder than you imagine. That idea is a bit stupid and won't work/more problems than benefits