[1] No, the PCV is NOT closed at idle when working correctly. At idle, it has a small gap to let air pass.
[2] The breather arrangement is not perfect, but it would not cause your 'problem' & I wouldn't bother changing it.
[3] Your 'problem' could have a few causes:
- when you removed & blocked the PCV, you removed a SMALL amount of idle air that the carb needs to idle with. I would have expected a drop in idle rpm, but not the engine running very rough & stalling....which leads to the next possibility.
- the cam. If the cam is 'sizeable' & has low vac at idle, the PCV may not seat at idle..& stay open. So then, a large amount of air FOR IDLE is being supplied by the PCV & then when it is removed the engine stalls because it has lost a large amount of the idle air.
- what rpm does the engine idle at? Rough or smooth idle? Do you have the cam specs?
- measure the vac at idle.