Stability of crystal and crystal-less oscillator transmitters
Your calculations are correct, according to the data sheet and using their ppm figures the actual frequency of 433 MHz could be out by up to 108.25 kHz.


  Does this look like enough for the FSK receiver not to be able to tell
  the signal any longer ?


No, I think a reasonably well designed FSK receiver should be able to lock on to anything in the realm of 433 MHz +/- 1 MHz. The FM demodulator should be designed to exceed the full range of the oscillator offset plus the FM deviation. I don't see this as difficult. Then you have a data slicer after the demodulator and you should be fine.

Remember that the drift is slow moving and a data slicer's speed at traversing across the full receiver demodulated output range will be many, many times faster.


  A few tips on how to program the receiver


I have no idea what you mean.


  whether or not I can get away with a non-crystal-less SiLabs paired
  receiver would help


"non-crystal-less" - what are you actually asking here?