RX Errors are under the radio or RAD tab on the telemetry viewer in both Mission Planner and APM2. The image below is from APM2 on a Mac, but I just looked at MissionPlanner on my PC and it’s pretty much the same.
I’m going to defer the Crossfire failsafe question to someone who knows more about it. I’ve never used it so I don’t want to give you bad advice.
Interference from your VTX is possible. I know from other experiences that 1.3ghz VTX systems will have issues with 900MHz RC systems like Crossfire. If everything check out okay on the ground then you could try a LOS flight with the VTX disabled. Then compare the RSSI telemetry values in the log between the flights and see if things improved.
I think you should be able to run an autotune on that plane just fine. I go full stick deflection and hold it for about a second, then full opposite deflection. At least 20 cycles for pitch, and 20 cycles for roll. If someone has some values to give you a head start that’s great but you’d still need to fine tune it for your build.