Regarding Observation 3222791 … Clearly it’s a valid reception, per the AX.25 frames decoded with the SiriusSat callsign- but how do we account for the odd frequency shift in the middle of the sequence? An error in Doppler calculation?
This was probably one of the last transmissions of the satellite before re-enters. At that stage the orbit of the satellite changes a lot and the TLE we get from our external sources, mainly space-track.org, are accurate only of a couple of minuter to couple of hours. So, the shift you observe is a result of non-accurate which results in wrong Doppler calculations.
In the past we have followed some missions before re-entry and generated our TLE sets based on our observations with pretty successful results. Check for example the Bobcat-1 story in NG-14/ELaNa 31 Launch and Deployment Information - #96 by Kevin or RamSat one in RamSat mission progress - #152 by pethornton.