Well, you're right about those brief moments of cognitive thought, as this is all coming back to me slowly.
The primary runs 24/7, with a Taco 007, iirc, which is putting out about 19gpm or so. My zones were fitted with variable speed taco pumps set to the lowest setting which is ~7gpm, so even if I have calls from both zones at once I have sufficient flow in the primary to make the system work properly.
And truth be told, I've never had a problem with heat calls to the zones, in fact the new piping scheme means zone 2 (upstairs) has all the radiators coming up to temp at the same time (and I thank you for pointing me in the right direction to make that happen). Zone one has issues because of a mismatch in radiator sizing, and will have to be addressed by zone valves, I think, at some later date. However, since the radiator output of the downstairs far outstrips the heat loss for the entire house, this hasn't been a problem in terms of comfort.
What has been at issue is the flow through the boilermate, or more accurately, the use of the OB as a way to increase the heated mass of the system thereby reducing thermal shock on the boiler because of all the cold water flooding back from the radiators. I had also hoped to decrease the recovery time of the WL 110, which in high mass systems seems to be very long indeed. That hasn't seemed to work, and in fact I haven't seen much change in performance since I replumbed the system.
What I have seen on more than one occasion is the high temperature limit on the boilermate go off, which would only happen if the primary loop was cycling through the boilermate instead of the OB. Thinking like water, I assumed this was because once the flow started through the boilermate, it would prefer that route instead of having to once again push against that 320lb mass of water in the OB.
Long story short, feeding the boiler mate from the primary AFTER it leaves the OB would address that issue, if indeed that was the cause of the problem.