Even if there was a constant idle feed port on the secondary side, below the slot(like the 3310 has)....... if the channel in the main body goes straight through from the air bleed to the transfer slot well in the base plate........how much fuel could possibly be pulled from the rear idle well with that set up?

Frankly, I don’t see how the rear idle circuit is functional at all(at idle) with it feeding only the slots, which are well above the vacuum source.

However, making that circuit functional at idle is just going to make it richer...... and from what you said previously...... that doesn’t seem to be what you’re looking for.

Adding secondary idle fuel would be used when you couldn’t get a decent idle from the primary side idle circuit, and/or you’re way out on the mixture screws and they were basically ineffective and the idle was still lean.

Since you’re only out 1-1/4 turns on the mixture screws....... it doesn’t seem like you need more idle fuel.

My guess is, the added area of having 4 discharge ports pulling on the ifr is a reason why it’s working with such a small ifr.
Similarly to how using boosters with larger feed holes(all else being equal) will be richer than boosters with smaller feed holes...... when using the same jet size.