I have recently used fallow deer back fat to start a venison pie, that is to fry up the leeks and onions, and it did give the pie extra flavour. This may not be to everyone's liking, but I did like it. I think deer fat works for cooking but maybe use it sparingly.
I have yet to try to eat deer bone marrow but look forward to trying. The stock made from deer bones is excellent. Roast the cut bones, then boil clean.
I have saved some deer ribs. This is a new idea too I have not seen before. Maybe there is a good reason? But it is worth a try.