I still can't fathom why Galen Erso didn't just send the message about the weakness. I mean, did he? He already told them the reactor had a weakness. Was that all he knew? If so, why did the Rebels so desperately need the plans? Did he know about the exhaust port? If he did, just put it in the message. If the message got intercepted, the Imperials could have just removed/reviewed the file from Scarif archives anyways, so all would be lost regardless. If they didn't get the message, boom instant success. This was an unclear and distracting point.
If he put it in the message and the message was intercepted, the weak point would be gone, not just the plans. Knowing there is a weak point is enough to go and find it. Maybe not that week, but certainly given enough time.
I don't buy it. The message was an all or nothing, as it were.
IF the Rebels get the message without Imperial interception, they're told to go to Scarif and get the plans. This forces them into a near impossible mission.
IF the Rebels get the message with Imperial interception, the Imps will just remove the file from Scarif and do their own analysis. They may or may not find the weakness (but if the Rebels could find it in a few hours surely the Imps would too), but the Rebels will never find it because they'll never see the plans.
So sending an ambiguous "go look" message accomplishes nothing, because even if it is intercepted the hope is entirely lost. So you might as well go for an "all or nothing" message and just pray it's not intercepted, because even the interception of a guarded vague message would have killed the mission.
He engineered the flaw in the reactor so that "any pressurized explosion to the reactor module will set off a chain reaction that will destroy the entire station", but he didn't know how to make said explosion happen. That's precisely why he advised the Rebels to get the plans - they have to localize the reactor and find a way to deliver explosices - be it via spies, boarding or torpedoes down the secondary exhaust port.