Checking on them every 15 minutes is not, to my mind, sufficient. If any other passenger were to have a pop at them in the meantime while they are defenseless, the airline would be held liable. You also want to ensure that they do not quietly work themselves lose when nobody is watching. Most restraint devices work pretty well, but are not foolproof. I would advocate what I said earlier - assigning one crew member to them for the remainder of the flight.
On the question of the use of the toilet, that also depends on the individual circumstances, the nature of the restraint device, how convincing the passenger is in persuading the crew he's calmed down, etc. etc., but I have every sympathy with the captain's position in this case, and "p*ss in situ" is the direction in which I would err every time.
Since many such cases tend to be aggravated by drink, use of the toilet is going to be more of a problem than it would with your average passenger who hasn't indulged himself liberally with the electric tinctures. So it can be expected to be a problem more often that you might imagine.
My final thought on such matters is that, when other passengers see the sort of measures that an airline is prepared to take to secure the safety and comfort of all passengers, it can only have a good effect. Firstly, it inspires rather more confidence in your airline, which is therefore good for ticket sales over your rivals, but also it will act as a deterrent for anyone contemplating the same sort of behaviour, who will realise that they cannot get away with it, and also realise the consequences.