1) The behavior of the Automatic IEEE 802.15.4 Operating Modes added by the firmware is explained in this document:
2) Yes - this is optional. (depending on adi,report-ack-frames-enable) When I last looked the kernel 802.15.4 layers didn't do anything with the ACK. For driver all possible device tree options please see here: bindings/net/ieee802154/adf7242.txt
3) There is the AUTO Status see here drivers/net/ieee802154/adf7242.c#n241 It'll give you the indication if the packet was successfully transmitted or not.