So what the heck was Galen expecting the Rebels to do if somebody built a chain link fence over the exhaust port? Send infiltrators all the way to the core on a suicide mission to destroy it?

I don't think he knew anything about the exhaust port. He just know what he had done to the reactor. That's why he was hoping th rebels could find some way to exploit it when analyzing the plans.

So what the heck was Galen expecting the Rebels to do if somebody built a chain link fence over the exhaust port? Send infiltrators all the way to the core on a suicide mission to destroy it?

And good luck finding a chain link able to stop an anti-ship weapon which can break a light cruiser in half.