How to cook fallow deer ribs?
You will not succeed in slow cooking your deer to a nice, flavourful dish.  There is not enough collagen or fat to do so; this is generally true of wild game.  It has occasionally worked for me with rump or brisket from an older moose, but even then, the rump would have been better ground.

You appear to have the backstrap (loin), which is one of the two best cuts for steaks (the other being the tenderloin).  I would suggest that you make medallions (e.g. venison steak Diane).  Also by the same author here is a general discussion of deer steaks.

If you really do want to roast it, it should be barded (surrounded in fat such as fatback or bacon) or larded (strips of fat threaded through it).  I prefer larding as barding reduces the Maillard reaction.  In any case, it should be cooked quickly.  E.g., an interesting recipe - which I have not tried - is here at Serious Eats combines roasting with post roasting browning.  I think this is as slow as you can do it.

Finally, a hacksaw with a medium blade can work as a meat saw if you want to try chops.  A wood saw can sort of do the job, but the coarser teeth tend to produce an excess of bone splinters.

Best of luck and don't forget to use your previous piece to make a great brown stock.