Some kind of "magnetic direction indicator" is required (for anything but daytime VFR). If you don't have the magnetometer, you will need some type of compass.
GRT's magnetometer is very nice and installation is easy. Their "older" magnetometer was pretty critical on orientation. Th new one is +/- 60° up or down.
The major problem with putting a magnetometer in the EFIS is there too much interference around the instrument panel location. It would be very difficult, if even possible, to "correct" for deviation.
CFR 91.205. That is not correct.
Ever see a Piper Cub J3 instrument panel. That is day VFR. Compass was #1 Nav instruments (and your eyeballs, map, watch) from the most early days of flying before GPS was even glimmer in anyone's eyes. Even A-N "Radio Range", ADF, VOR, LOC don't work unless you know our heading.
(b) Visual-flight rules (day). For VFR flight during the day, the following instruments and equipment are required:
(1) Airspeed indicator.
(2) Altimeter.
(3
) Magnetic direction indicator.
(4) Tachometer for each engine.
(5) Oil pressure gauge for each engine using pressure system.
(6) Temperature gauge for each liquid-cooled engine.
(7) Oil temperature gauge for each air-cooled engine.
(8) Manifold pressure gauge for each altitude engine.
Another 9 items not related to instrument panel for day VFR. For night VFR there are another 6 items.
You are right interference right behind the engine, electrical wiring, steel roil bar (slider). It is interesting you say the allowances for magnetometer is less critical or stringent for the digital unit. The manual must not be updated because it shows some strict orientation requirements.
Speaking of back up I have the GRT EIS separate display for engine. For airspeed, altitude mag direction in the past, early days of EFIS certified or EAB planes used mechanical, pitot/static instruments, and wet compass as back up for bare min flight instruments (day VFR). Since I only have one display I plan on a back up small EFIS. There are many non-NAV EFIS that will do the trick to get you home legally and safely. Even the jetliners have canned any mechanical gauges. The Standby instruments are glass. They run on essential DC buss, and also give standby attitude and NAV. However the primary EFIS is redundant to beat the band, and can reconfigure to restore information on both Captains and F/O's side. I don't need that for the RV-7. But if I put in a IFR GPS source I will for sure get more redundancy.
As far as accepting of EFIS magnetometer as meeting the FAR requirement, that was controversial in the past. Many designees and FAA folks balked at the idea of not having a wet compass. I believe that has been resolved and a wet compass is not needed. However I am 100% sure you will find some out there that want to see that jelly jar (or vertical mag compass) no batteries required installed, despite your dual AHAR, dual magnetometer, AHRS, GPS.... ADS-B weather or XM weather. Ha ha. They want that jelly jar compass.
Great Grandfather of Day VFR.. A simpler time. Before my day but I still make students fly by dead reckoning and pilotage, in part it's fun, and FAA still requires it.