United should return the GPU. LH doesn't really know about GPUs and I wouldn't expect them to instruct UA to return it.
I'm not sure whether €1000 is the right compensation for a downgrade under EU261/2004 -- seems like that could be lowball. I think it might be 75% of fare paid, even if you used upgrade instruments? Good luck figuring that out (could be very lucrative) (the BA forum knows this pretty well because F=>CW is not altogether unseen and pays very well).
Whichever mileage program you credit the journey to (MileagePlus?) should handle giving credit for D instead of I but may not be automatic. Good luck, hope you saved your original e-ticket receipt and boarding pass just in case.