Cross-talk between the earcup drivers is being recognized as an important issue for headphones. Going back a few pages, I referred to Mitchell's measurements of this. He also noted that with some wiring setups you can get electrical cross talk as well. See p26 of
http://www.politicalavenue.com/108642/US-MAGAZINES/Hi-Fi News - July 2016.pdf in HiFi News and Record Review. I assume that electrical crossfeed is worst with common ground wiring found in most dynamic phones, but not Stax electrostatics, which use separate grounds for each channel. Sennheisser uses its mechanical damping material solely in the headband of the HD 800, according to their advertising. I see that their new and hyper expensive H1 uses a large swath of padding under its headband, which I assume also mechanically dampens the headband to prevent mechanical crosstalk. Similarly there is one set of B& W and Audioquest phones that specify that their use of damping in the earcups addresses the crosstalk problem. I think they are missing part of the issue though which is that mechanical vibrations will affect the same side earcup even if there is no headband. Thus I have found very large improvements of the sound of earbuds and IEMs by adding sorbothane damping and these of course have no headband at all
You raise the question about crosstalk through bone conduction in the skull. Certainly hearing can take place, skipping the ear and directly stimulating the the skull .or jawbone There are hearing aids designed to be used by bone conduction for people who get limited benefit from conventional hearing aids. There are also a number of sports headphones which some people like because they keep drivers from blocking the ear.
I would think that if the hard surface of the headband touches the skull you could indeed get bone conducted crossfeed. However padding should stop most transmission to the skull. But as I noted in this thread only some kinds of padding will reduce mechanical vibrations to the same-side earcup or create crossfeed to the opposite earcup through the headband. So far my experiments say high density (i.e. 70 duro) thick ( 1/4 to 1/2 inch) sorbothane applied with self-stick tape or better glues such as the Lord line are the most effective.