Welcome schermerberger ...
I don't take much notice of muting behind the
slide, I don't often hear anything too ugly coming from back there, but I know other players do. I don't know why I don't have that problem of unwanted harmonics ringing out. I just checked then and I can keep all fingers off the strings and still get no real problems.
I think my fingers must be pudging out a bit as I age as there is no real loosness anymore in my slides. They're not snug (yet) but there' very little air in there. I think I use the weight more than anything else for vibrato, a sort of upside down pendulum action. The slides I use have quite a heft to them. It's a relaxed thing, letting the slide do the work.