Yes. Pressurising the sump should certainly help. What a performance for something that should not occur. I had not thought of going that far. Don't be surprised if you end up pushing some oil through the rear crank seal ~ the "rope" seals are not particularly effective. Keep the car absolutely level if you do this. Not nose up ~ we don't want oil at the rope seal. Desperate times call for desperate measures.

The pump must be worn or something sludged up. Your new oil will be a high detergent product & likely to loosen up a lot of gunk. Suggest you do another oil change at 1000 miles to drain all the suspended sludge out. You do not want a clogged oil filter to push the the engine into bypass.

Block the breather at the pipe that goes to the aircleaner/carb inlet. No need to remove the breather assembly from the front of head/chain case.