Multiple up conversion do nothing to help the modulation index. It will still be the same low deviation as you started with.
No, there is only one crystal, used for a reference. That is why you use a programmable PLL to select the desired channel
Stereo encoding is very easy. You need a 38 KHz signal, and a divide by two you generate the 19 KHz pilot. The stereo encoder circuit is dirt simple these days.
Let us know if you ever get it to work properly.