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It appears that you are doing your homework and maybe you will come up with something really nice to share here.
Regarding crystal accuracy, the frequency error in Hz. is divided down through a divider but the percent error is not. If the error is .05% that will be the error after division.
Also, be clear about frequency error and frequency stability. Usually the frequency error on a communications crystal can be “netted” to zero with an adjustable cap. but we are also interested in the long term stability of the oscillator so we don’t get unacceptable drift with time and temperature. The pros use a crystal oven usually at 75C to hold the crystal temperature constant yet even then circuit changes will pull the frequency. You won’t need this for part 15 AM though.
A side note: I periodically check the frequency of my SSTRAN on AM and my FM-25A on FM using a NITSC traceable counter. The SSTRAN is always within +/- 10 Hz. and the FM-25A within +/- 200 Hz. Not bad at all! I have not had to adjust either one after the initial setup.