Currently there are no options to shorten the infoWindow tail.
You are correct that pixelOffset is not what you are looking for. It is the equivalent of setting the infoWindowAnchor offset resulting in the same length tail, but pointing at a point offset (presumably downscreen) from your icon. So you can move the window using this approach (as long as you use map.openInfoWindowHtml, not supported in marker.openInfoWindowHtml), but the tail pointing to something below your icon looks pretty average.
I hope I have missed something here and someone can correct me.
Alternatively, it would be great if this feature was implemented. I feel like pixelOffset should do this, as you can already offset the infoWindowAnchor when you create the icon.