Who said anything about the defenders being the ones doing the treachery? More than just the garrison lives in the fort.
As for a few other comments, treachery has always been a big part of siege warfare, whether it's bribing someone to open a gate, like Bohemond did at the siege of Antioch in 1097, faking orders of surrender, as Baibar did at Krak des Chevaliers in 1271, using secret agents to infiltrate the city prior to approach, like the Mongols liked to do, or someone inside the castle flat out betraying the defenders, as a monk did in 1676 at Solovetsky Monastery. Some form of deception or treachery should be in the game's model of siege warfare, unless you want to argue that this is covered by the die rolls (fill in your own story why you had an early breach). And it seems to me that the nearly entirely useless espionage idea set is a good way to model it.
You could even expand on the idea in other ways than making a breach, it could be a % to siege ability, making it faster and adding another way to counter act the defensive bonuses than can get pretty high late game. Another option is to increase the odds of a successful phase. Your insiders "show" you the best way to make a breach, sabatage supplies, encourage desertion, etc. I feel like you could run with this pretty well.
As for those saying that if you are caught fabricating, that the process should end immediately.(without you getting a claim) how would you balance it? Several issues come to mind. How do you expand as odds are you will be caught, and the odds increase in the mid to late game. Do you rely on missions? Outside of the ones against rivals and ones given a country(like france getting one on savoy) you wont get many. and how would it effect a "non popular" country(meaning you don't have many events or missions specific like other countries. ex france).
How will wars be when you can only take one or two provinces with claims, and anything extra will hurt you in AE and MP. What about other CBs? If you drastically make conquest a lot harder via the change to how you get claims, that will make other CBs like the one in religious very big.(almost OP compared to the "new" form of way to get conquest claims) Do you have to wait for the CB at the end of expansion? or imperialism and nationalism before you can mass expand?
These are just something that come to mind. I would hope anyone proposing changes to it can come up with a way to somehow balance it all out. Otherwise you may just end up with a nerfed way to get claims, drastically slowing any expansion.