A ewe of that age, yes, long and slow cooking is the way to go about it.
Chops want longer slower cooking too; bake them in an enclosed dish on a bed of veg with some wine and/or stock in a moderate oven for an hour or so. They'll come to no harm if left longer. If you want them crispy on the outside, put them on a baking sheet on their own to finish for 10 minutes at the end.
You can also cook the chops using any recipe which parcels the chops and bakes them in a foil wrapper so long as they are cooked reasonably slowly for a minimum of an hour.
You can also use variations on a Navarin of Lamb recipe with any of the small cuts - baste the meat in a very little fat (the fat you tipped off your last mutton dish works brilliantly), remove the meat, soften some onions, add 'fingers' of carrot, turnip, parsnip and cook until golden, sprinkle a little sugar and cook until glazed, add a little flour, stir and cook for a minute or so, add stock and mix, return the meat. You want the stock to come half-way up the meat. Cook, covered, for 45 mins in a moderate oven, or all day in a slow cooker. Turn the meat over and cook a little longer. Then reduce the juice to about 1/3 its volume. Professionals discard the veg it was cooked with, I eat the lot.
However you cook an old girl, there will be lots of fat, so cook in ways that make it easy to drain, skim, and so on. The fat off my old girls has always been deep yellow verging on sunset orange, so easy to see!
Mince will be the best mince you have ever had, you will never want to make lasagne with any other kind of mince ever again - but again, make sure you allow some settling time and skim the mince mix before combining the layers into the finished lasagne.
You probably will still get some sunset orange pooling on the plate when you dish up - I think it looks very attractive but you can always tip it off the plates before serving if it bothers you, or your guests.
(I have carefully not used the word mutton as a French friend told me that 'mutton' or 'mouton' means the flesh of the 18 to 24-month old wether, and as such much of what I have written above does not apply. A 2nd-summer hill sheep can be cooked as lamb but has the depth of flavour of mutton - and usually not the fat.)