I have 2 theories. One is that it could have been difficult to tell that the moving air you felt was actually coming out of the vent or into the vent. Assuming that you accurately determined the air was coming out of the vent, my other theory is that your PCV valve was jammed shut. The PCV valve is a one way check valve that should only let air come from behind the valve cover and into the intake manifold when it’s under vacuum. In order to replace that air, there MUST be inward flow from the hot side. If both of the above are incorrect, then my last theory would be that one or more of your 4 pistons are making incredibly poor compression and the blow by is giving the crankcase enough positive air pressure to need to vent out of both openings on the valve cover. Without being able to diagnose your car in person it’s hard to say what caused your experience.

The PCV valve is the same part for all 1989-2004 Miatas. The check valve allows airflow into the intake from the VC and not the other way around. In order to facilitate moisture evacuation, this pulls air in on the hot side. Which is behind the MAF in all OEM applications. Without a high resolution log mapping throttle, intake pressure, and AFR data overlaying, it would be very hard to confirm this by just watching an AFR gauge. It wouldn’t make much difference under and sort of load but again it can cause drivability stumbles and hesitates going on and off of a fully closed throttle for milder stop and go traffic, parking lot, or rolling onto throttle while in gear on the highway type of driving situations.

I do not have first had experience running an ITB setup but I'd say off of my understanding of the theory alone, if you don't have a vacuum source, I would just route both Valve cover outlets to a vented catch can. If you are running a vacuum block I would route a more traditional PCV circuity on the intake side of the VC with a sealed catch can between the PCV and the vacuum block. Then I would run the exhaust side breather to a vented catch can, You still don't want a small cone filter on the hot side because there is always the potential that the engine may lose some oil out of it and you don't need to be dumping oil onto your exhaust header. It's important to note in the instance of ITBs, that the metering and air/fuel mix issues I focus on in this video would not apply as an ITB setup absolutely cannot be ran on an OEM ECU.

I can’t say you are wrong but these are 35 year old ECU’s. I don’t think they were that smart back then

I’d route with a different hose. Exposed cam gears do look pretty cool but definitely run the factory under tray if you do it because you don’t want a pebble to fly up and get into the timing area.