So, the two questions are - does the PCV allow a certain amount past at idle?
Yes. The PCV is vacuum-operated where vacuum is opposed by spring tension. High vacuum pulls the metering valve into position that restricts--but does not eliminate--air flow.
Under positive pressure (backfire) the metering valve inside the PCV should block flow--no flames in the crankcase.
Sounds like you had a backfire AND a failed PCV valve, which ignited the fumes in the crankcase.
And, are there any negative consequences to blanking off the passenger's side one if I have to?
Yes. You're blocking the vent that allows fresh air into the crankcase. The fresh air replaces the crankcase fumes drawn out by the PCV. Thus there is continual air FLOW through the crankcase. With no fresh air inlet, you'll develop a vacuum in the crankcase which will be less effective in removing moisture and fumes. And during times of high blowby-low vacuum, you'll pressurize the crankcase with blowby leading to oil leakage from oil-flooded seals, pushed-out valve cover gaskets, and similar undesired venting.
You MUST retain the PCV suction side, AND the PCV venting side of the system.
What you would not have to do, is connect the PCV vent to the inlet of the air-intake system for the throttle body/fuel mixer. You'll have to tune the fuel mixer a little richer to compensate.
Mind you, not having the vent side connected to the air stream for the throttle body/fuel mixer will (illegally) increase the hydrocarbon emissions of the engine via the fumes escaping from the PCV vent into the atmosphere under high-blowby/low vacuum conditions.
Be sure to put a filter of some sort on the vent side of the PCV system, when you disconnect it from the air intake side of the throttle body/fuel mixer tubing.
As said, if you can connect the PCV vent upstream of the fuel mixer, but downstream of the MAF, you keep all the benefits with none of the drawbacks.
PCV marketing stinks. There's no chart or guide to flow rates, spring tensions, vacuum needed to pull the metering valve, etc.
But PCV flow rates are in general very low. I'm surprised that you're having problems with it--PCV is only about 5 or 7 cfm.
Try a different part number PCV valve. Perhaps one intended for a four- or six-cylinder?
And as I said earlier, I think the PCV you have is defective, there shouldn't be any flames in the crankcase to begin with. Which is why the manufacturer of the fuel mixer allows a combustible mixture in there, when liability lawyers would eat them up in court for damages if it was actually dangerous.
"I" would be studying OEM propane-powered engines--forklifts, for example--to see what solutions they've come up with.