If you go back to the too tight setup does it go back to going 30? I mean are you sure the 15mph limitation is clutch? If the clutch was spinning too much you'd hear it continue to rev up but without going any faster.
One check, and only do this to check, is pull the starter while riding. that will force the clutches to engage. You don't want to do this for a long time but if it makes your bike get up over 15mph then agreed, you've determined it is a clutch engagement issue. If no change though, I think you'll need to look elsewhere, timing, decomp, jetting, carb problems, plug or points etc
as far as the rear wheel spinning, the clutches always have some resistance wih the oil and whatever. as long as it idles without having to be a high idle, and as long as it's not tugging you along at a stop, you are OK. Also, be sure you've got the wavy spring washers with each pad. you may be able to anneal those just hitting them with a torch or may benefit from replacing those to help push everything apart better and disengage at idle. But I don't think that'll be necessary.