AirRouter HP installtion issues

We just encountered an issue with "older" AirRouter units MFG Codes 1503G and 1436G - Came with UBNT XM 5.5.8 Firmware.  Attempted to update to UBNT XM 5.5.11 (from the UBNT AirRouter Firmware area) and received a "firmware image check failed. error code -6).  One of these units was purchased used and the other one was fresh "new in the box" from a dealer delivered today.

I usually like to have the latest compatible UBNT firmware on fresh units before flashing the AREDN images. The UBOOT test checks these units to be GOOD GOOD, so we proceeded to install the latest 2016 Beta01 firmware without any issues.

Our previously purchased "new" AirRouters over the past few weeks had manufacture dates of 1546G, and 1545G with UBNT XM 5.6x firmware and they "downgraded" to 5.5.11 without issue.  So there seems to be some minor difference in the boards between these dates and the units that arrived with 5.5.8 installed.

I can't seem to find the "old" 5.5.10 UBNT firmware available under the AirRouter areas on the UBNT site.

Apparently no matter, but my two questions raised:

1.  Is there any advantage or need to load the "latest compatible"  UBNT firmware on a unit before flashing to AREDN? 
2.  Is the original UBNT firmware totally overwritten by the AREDN Firmware?

Thanks! - T.D.