I know this is drifting a bit into a tagging discussion. The area I am unhappy with is explicitly the situation like the two examples above. Not the situation in countryside.
I agree that the underpass/u-turns for motorcycles exist and probably your proposed tagging will work here.
I am not certain about how suitable the path key is here. Could also be a service road or a link, like secondary_link, motorcar=no.
I am also uncertain whether we should tag on the example of the Ping bridge above the motorcycle lane as individual way in OSM. Then we would have to add another pair for the footway. I did never dive too deep into that sidewalk/parking lane tagging thing. So I am uncertain what the current tagging standard is here.
How about splitting the discussion here? The table looks good for me for most scenarios. And we discuss the small aspect of dedicated lanes in urban context separately?