I've been trying to figure out how to use a composite mode such as multiply yet control the opacity of the effect. In particular, I need to overlay grain (using screen mode) on top of some footage but need to control the visibility of the grain footage. Is there a method that'll allow this in Kdenlive directly without destroying the composting method's blending effect?

It is not totally clear what you are trying to do here. Is it one track with the grain effect applied being composited over the top of another track?
Do you have a composite transition between the two tracks? You should. And then you just adjust the opacity of the composite transition.
I don't think you want the screen transition as this is for footage shot against a black background.