I understand the points you're making, I see the gist behind all of them and agree with some of them. I don't pretend to have all the answers! But I think there's a lot more going on besides.
E.g. The size of the holes for gas to enter the air must play a part, all other things being the same a single pin prick size hole would still generate the same vacuum signal as a larger hole if we were to connect a pressure sensor. But if we have a reducer connected to a pin prick sized hole and all gas flow entering the engine has to pass through the pin prick the pin prick will limit restrict gas flow from the reducer, there will be a pressure drop between the reducer and the outlet side of the pin prick which sees a drop in signal at the reducer. With a pin prick sized hole it probably wouldn't flow enough gas or petrol even if we fed the pin prick with liquid at 5 bar differential pressure, never mind the small fraction of 1 bar with vapour that mixer systems operate with.
We can see manifold pressure on a lot of engines because they have manifold pressure sensors fitted which we can read via OBD. At full throttle with no restriction (such as a mixer) fitted the manifold should reach 1 bar at WOT even at bhp max. With a well matched mixer fitted the map might still reach (say) 0.95 bar at WOT at the rpm where the standard engine makes bhp max, from this we can take it that a well matched mixer setup can deliver enough gas for correct (suitably rich) mixture at WOT across the rpm range for a manifold pressure drop of only 0.05bar. Not sure how much exposure to manifold vacuum plays it's part in signal but it seems likely that speed density also plays a part with a well designed mixer.
It would be interesting to connect sensitive pressure sensors to the manifold and to the signal port on various mixers on various installs to see how the readings compare. Could run tests with the engine running on petrol (to check vacuum signal when the reducer isn't supplying gas) and again when the engine is running on gas (check vacuum signal when the reducer is supplying gas).
One of my points in my previous post concerns this effect https://en.wikipedia.org/wiki/File:VenturiFlow.png
Imagine there were no water in the side pipe of this venturi tube, there would be a flow of air through the side tube, how that would effect your 60/40 calculation using a single reducer I'm not sure, with 2 reducers independently feeding your 2 series mixers I expect it would effect your 60/40 calculation a lot less.
There are 'shield type' mixers which are effectively just a post/restriction over only a small area of an intake pipe with the gas outlet on the exit side behind the shield.
I've driven thousands of miles in a Jeep using a mixer I made from a baked beans can! Removed the lid and made a hole in the bottom, increased the size of the hole to be as big as I could get it while still getting enough signal. Gas entry point just an open ended brass fitting screwed through the side, it ran great lol.