What can I do with the meat and fat trimmed from the top of lamb ribs?
I always save all my scraps to make stock.  I'm not sure what you would do with lamb stock, but it would probably make a good sauce to use on lamb.  The fat that renders out is also useful for future cooking of whatever it came from (duck fat for duck confit, for example).  

I just throw all the scraps into a slow cooker with celery, carrot, and onion (veggies are optional) and let it cook for a day (or two).  You can pick out the meat if you like (after a day it will be easy to separate) and use it for anything you might use shredded lamb for (pot pie, shepherd's pie, etc.).

I use rendered chicken or turkey fat instead of butter for making pot pie dough, to cook more chicken in, or for matzo balls, and beef fat instead of butter for making gravy, replacing the butter in a beurre manié (See https://en.wikipedia.org/wiki/Beurre_mani%C3%A9).