The plate will separate the half’s on the intake. It’s impossible for the exhaust gas to travel to under the intake. The EGR valve has a vacuum operated valve that is open under idle to mid-ish throttle. It will close under a heavy throttle, approximately 3/4 of the way down. I don’t remember the exact reading I took on my EGR valve, but it was something like 9-11 inches.
When the WGR valve is open, it allows the exhaust gas to travel from one side of the intake to the other side. The plate blocks it off, period. Unless there is a bad gasket under it. Even then, it’s a very low amount.