I can reconfirm that non-commissionable rates earn points still - almost all my corporate rates, and quite a lot of special offers like the one you see note non-comissionable. So you will definitely get elite nights and points (but no cashback of course)

Whether the 28 euros is worth the extra points: It is a good deal vs buying direct (in the current 45% bonus sale that would cost you about USD50 for those points), but I would be careful unless you have a specific redemption in mind to use them in the near future.

My concerns here would be, just double check when you say "extra 6k per two night stay", where you imply multiple rooms: I tend to find these offers have some hidden small print somewhere that says "only the first room/first member gets credit", so if booking multiple rooms, to guarantee those bonus points per room either discuss in advance with the hotel, or book as separate reservations with separate Marriott accounts, and then consolidate the points as needed.