How to prepare mutton so it's not tough?
Lamb shoulder requires only one thing - time.

The marinade would work wonders I'm sure. However lamb shoulder needs a long slow cook. I would roast it in one piece then carve it for serving. I suggest two methods:


Slow roast. Perhaps a 150C heat, maybe 5 - 6 hours. It's ready when you can pinch the meat off the bone between finger and thumb. Make sure you allow it to rest for a good hour.
Part-braise. A Greek family taught me a lovely recipe which involves using your spices slackened down with enough water to cover the base of your roasting dish, then continually basting the lamb as you go along. Roast in about a 170C oven for about 2 hours, covering with foil for the first hour to build up steam.