In the way of War Stories....re instrument flying in helicopters I have this to offer.
As part of my US Army Helicopter Instrument flying course at that wonderful Alabama Resort area known as Fort Rucker....using Huey's....we at some point had to enjoy a flight into Knox Heliport by executing an NDB Approach with overhead Hold.
Piece of cake right.....No Gyros, Fixed Card, unstablized single engine helicopter with no RadAlt.
That it was in IMC proved to me my Instructor was near suicidal and had real intent to take two others with him in the process.
That it was I doing the flying....is what convinced me of that.
Artificial Horizons are very nice kit...and have their uses....but they in my definition (if not in the usual context/definition) are "Secondary" instruments.
I am of the school.....that a single Attitude Indicator is a necessity but a Backup is pure Gold should the first one lay down on you while in IMC conditions.
A test I used while Instructing Instrument Flight.....was to use Unusual Attitude Recovery Exercises to determine what technique the Trainee used for controlling the aircraft.
I did the usual, Head down, Eyes Closed, wiggle the sticks all about and add some varying kinds of G Loads.....but that was just to disguise my setting a Five Degree wing low attitude on the Student's Attitude Indicator without being seen.
Then I gave control back while in a very minor non-level, out of yaw trim condition and sat back and observed the recovery to level flight.....and in some cases could have run out fuel waiting for that to happen.
Some Pilots very quickly sorted out the Attitude Indicator was out of whack as they were flying the other instruments and using the AI as a Reference. They would set wings level, pitch attitude level....and then correct for the resulting variation and in a cycle or two....perhaps three...sorted it out.
Others....as mentioned NEVER would have gotten settled down.
As to moving one's Noggin about while "Scanning"....in the scenario above....looking at the Standby...and other Pilot's AI worked well too.
Just looking at two AI's....each showing something different...does not immediately tell you which one is the liar.
In some of the Accident Reports we read about now and then where airliners smack the ground at warp speed because of a loss of control due to instrument failure...it makes you wonder why the crew with at least three AI's could not sort out the problem very quickly.