My understanding is that there is still no way to have stemmed and un-stemmed slash voices in drum parts share the same voice. I am wondering why it is the case that these are not in the same voice to begin with. What is the advantage to having them in separate voices?
It’s simply how we decided to implement slash voices: they are either stemmed, or stemless. You can hide the stems of individual stemmed slashes using the Hide stem property in Engrave mode (or the Toggle Hide Stem command, to which you can assign a shortcut or trigger via the jump bar).