Theory on Unlocking the Other Oathgates.

[Argent he/him]

I wouldn't say "lightly scratched:"

Renarin stepped up and carefully—using his hand to pinch the weapon from above—placed the tip of the blade on the metal around the keyhole.He grunted as the Blade wouldn’t cut. He tried a little harder, and the metal resisted the Blade.
“Made of the same stuff!” Shallan said, growing excited.

 

 

I am calling it impervious. 

[Aether he/him]

Well, maybe not lightly scratched, but couldn't Shardplate yield the same resistance if you tried to just cut it slightly? Again, not saying it isn't impervious, but that excerpt is rather non-conclusive.

[hoser he/him]

It is by no means conclusive, but I try to look at intent.  What is the point of the extra words Brandon uses?

Well, maybe not lightly scratched, but couldn't Shardplate yield the same resistance if you tried to just cut it slightly? Again, not saying it isn't impervious, but that excerpt is rather non-conclusive.

He tried to cut hard enough to grunt (yeah, yeah, Renarin, but still (hoping Feather doesn't see this)), then harder and still nothing.  If it is like Shardplate and a harder impact would make an impression, then it seems like intentional misdirection.  If it is impervious, then he is giving us a clear hint with the extra detail about grunting and trying harder. 

[Dravs]

It's possible that the Heralds had no part in the Recreance, but instead showed up after it and only then did they lock the gates. 

 

This theory makes the most sense to me at the moment, as Talns blade would have been the only one they wouldn't have had access to, and therefore his oathgate would have been the only one that they could not lock.  The heralds seem to have access to fabrials that no one else does, so it would make sense that they would be more knowledgeable regarding their construction and control.

 

Now that they have Jezeriahs honourblade and his seat presumably being in Alethka, and with Kaladin going there in the next book, it would make sense that after the Everstorm transforms the parshendi Kaladin fights in a retreat to get everyone into the Palace as it is defensible, and at the last moment they work out how to activate the oathgate using the honourblade there and get everyone through.  That feels like a very Brandon thing to do.  Would make a great avalanche at the end of the next book.

 

As to why they were originally locked, I imagine the Shin either had a hand in it, or at least know the reasoning.  Urithiru is holy to them for some reason, and they have all of the existing honourblades.  Possibly they saw the betrayal of the radiants coming and locked them out?  They would have a means and motive to do it.

[dyring]

Jasnah may not have progressed enough to have a shardblade when she investigated the oathgates from the non-urithuru direction. Just to point it out. So it might just be the live shardblade thats missing.