Patrick, your observations about the reaction are correct. The interpretation of the data is due to Kinetics and the nature of chemical equillibria, no more.
Sulfite is NOT the cause of the synergism between HQ and Metol. As M&J says in the text, the rection of each is accelerated (they specifically refer to a Metol experiment as their major talking point).
So, the bottom line is that what you say will work, but the reasoning is different to a chemist as to why. Thats all. Sorry if my comments seemed otherwise to yours.
Superadditivity is an older word for synergy in some cases. It is still used in the MQ case.
Urea is hit and miss. Remember, it softens film so be careful. I would start at 1 g/l and go as high as perhaps 20. Remember that it can cause reticulation. Ethylendiamine sulfate will cause fog if misused. I would use no more than about 10 g/l and that is it. It will change the pH too, IIRC.
I am only bringing this up for experimenters to tinker with, so I'll have to say good luck. It is a chancy thing with both and that is why they are not used in production chemistry AFAIK.