How to achieve “fall off the bone” in a leg of lamb using sous vide

According to Kenji Alt's experiements (admittedly with beef), the major effect of the bone is insulation, which matters in a high-heat cooking environment, but not in sous-vide. See: Do Bones Add Favor To Meat

For sous-vide cooking, the bone adds little value, other than appearance.

It may also make it harder to put the product in your bag and seal it without a puncture.

Still, there should be no harm in leaving it on if you choose to. In his ultimate steak method, Kenji Alt does leave the bone on for the sous-vide process.

With all due respect to Chef Oliver, I do see some problems with the recipe he offers.

In The New Best Recipe Book, from Cook's illustrated in the section on beef roasts they note that when beef is roasted at above 250°F the internal temperature will rise by 18°F during the resting period, however when roasting below 250°F the internal temperature only increases by 3°-5°F. For this purpose lamb is comparable to beef and the result you experienced was to be expected. IMHO the oven temp of 400° is too high, and I would recommend going lower and slower. Try 225°F, and plan on it taking much longer to cook. Continue to roast in the oven until an internal temp of 140°F is reached. This technique will afford you a much more tender and juicy roast, even if it takes longer to get there.

You are correct in directing some attention to the stuffing as well. Where the recipe asks you to add lemon juice if it the stuffing is too dry, I would suggest adding lemon juice until the stuffing is 'very moist' (not sure how to quantify that for you).

Tying the twine too tight is unlikely to have had an impact, but your choice of basting wine may have. Go with something fruitier/sweeter in flavor.