Alright so you had replaced #2 and #3 from the diagram above and #2 on the diagram below, which is in deed the diverter valve I mentioned. BMW upgraded the design of the DV, which is the listed part number - 11658636606, and has removed the old rubber diaphragm that was known to fail. From my understanding the new design of this DV is much better than its previous predecessor as the holes in the piston assembly actually allow the pressure to build on the back side of the valve seal. This essentially allows it to expand and seal better than with an o-ring. I believe it should fit your MCS.
My concern now is that you possibly have a wastegate issue, since you replaced all other viable options. Thankfully we have a pretty helpful wastegate testing tech article that might be helpful to you. Check it out here:
Pelican Technical Article - Mini R56 - Engine Turbocharger Wastegate Testing. Let me know what you find out.
