Sixto,
I think the reason a straight 6 has that issue as opposed to a 4 is that the natural mode is within the rev range of the motor where in a 4 cylinder it is probably much higher. I'm not sure as it depends on allot of variables. This article explains the modes for a v8 and gives some good background info but I concede torsional harmonics get over my head pretty quick, not my area of specialty.
http://racingarticles.com/files/general-damper-article-2.pdf
Yeah, I got nothing other than my original theory to explain #1.
Didn't mean to get on the crankshaft torsion tangent.
Thanks,
Ashley