All your observations sound fair. If the filter is clogged up, over pressure builds up in the engine. This can blow gaskets and seals over time. I don't find it strange that it affected the oil pressure gauge.
Further, as the engine breathes a small amount of air through that filter, bypassing the carburettor, it's state actually affects the fuel mixture. That, and because the crank case is now ventilated, is probably why the engine runs better.
The hole in the carburettor is very small - did you pass a pin through to verify the passage?
I have a photo of my engine bay here, It doesn't show all the filter connections, but I think you can make it out. From your description, I'm not sure if you have turned it upside down?
http://gallery.dinsen.net/v/Murena/DSCF0024.JPG.html- Anders