If the mag is seeing 20-30% wow to answer your questions
Can the magnetometer be the cause of the red sensors (att and stab) and preventing the arming?
Yes, it will not arm until it's below mag alarm error
Same for satellites will not arm until it see enough.
Just changing the configuration to Basic Complementary, sensors turn green and i can arm the device. (Also removing the Postion Hold switch)
This because the FC does not use gps or mags in this mode that's why you get green
I forgot to mention, I was doing this inside my apartment. Can the magnetometer show this level of error because of this?
Don't calibrate the mag in apartment there is to much Magnetic Fields produced by the household wiring best to do it away from the building. Away from any mains power. Also when you do the mag calibration do it with all of the items on e.g vtx, hd camera, leds,
Are you using the small OP GPS? Where is the compass, in the GPS or on the Revo Board?
I use op8 with Revo on board mags with much success.
do you have a picture of the wiring and where the FC is placed?
Are there any high voltage wires near?
What is powering the fc?
Any magnets on the airframe e.g lid holding-wing joinings?
I have my wing 1meter with a revo, op8 gps, osd, airspeed sensor, hd camera, fpv system,
Fly's great.