I wasn't thinking master cylinder when I said that, but it wouldn't do any harm to strip the whole thing down from reservoir to where the slave acts on the arm just to be sure you have got all you need working right. Once that is eliminated then you can go back to trying other stuff. With a sensitive operator on the clutch pedal you may be able to just unbolt the slave from the housing and see how it behaves, but That may just result in a spurt of clutch fluid and the piston flying out, so be very careful!
And I am assuming you can get at it without having to get the bell housing off. I do not know this as mine is an auto. If you can't then it is Ministry of Sh!t Design time again!
Again, with DMFs I do not know if, if one is fubar, whether that would introduce slip. Commonsense says "no" to me ,but then there isn't much common sense in modern car design! If the DMF can move in and out to any degree at all when you push and pull on it then that may be your problem
It not being well sprung when you push against its springs in a radial fashion ought, common sense tells me, to just result in clutch take up being too sudden on the gearbox and engine, but you say you don't have that.
Tis puzzling that's for sure.