RHF76-052 covers 410-525MHz frequency band and 868-915MHz frequency band, RHF78-052 only covers 410-525MHz frequency band.
The factory firmware is the standard AT instruction set, and does not support secondary development.
The difference between LoRaWAN and LoRa is that LoRa is a technology, and LoRaWAN is a set of standard specifications, like the relationship between NB-IoT and 3GPP TR.