Gimmick may not be the right word, as I certainly didn't mean it to be derogatory. However, I think Bill Grover was a brilliant marketer and I see the "right-handed" gun being more a mark of distinction than anything that is practical enough to be worth all the extra work it took to make. If you find anyone who has seen a TLA but knows little of them, the one thing they all remember is that the guns are "backward" or "right-handed" or some such term. I think Grover made his guns this way to make his company stand out - and it worked.
Regarding improving Keith's gun, the improvements were minor. I would add to the metallurgy you listed the following improvements:
1. Caliber. Grover really had nothing to do with this, as the .44 Magnum wasn't around in 1929. But, I'm sure Keith would have chosen .44 Magnum for his "Last Word" had it been available. We see this in the fact that Keith virtually abandoned his #5 as his primary carry gun once the .44 Magnum came on the scene. Playing "what if" further, had Keith had a gun that could handle "Ruger-only" .45 Colt loads back in the 1920s, I bet the #5 would have been chambered in it instead of .44 Special or even .44 Magnum.
2. Sights. I think Grover's sights are an improvement over the original #5 sights, particularly the rear sight. Again, it isn't that Grover designed a better sight than Keith - he just had better options from which to choose than Keith did.
3. Springs. Replacing leaf springs with coil springs is definitely an improvement over the original. However, Keith himself said this, especially when referring to Rugers. Again, not a Grover innovation, but definitely an improvement over the original.