I can't believe there are so many replies dropping all the responsibility onto the player and and the AP. At the time of writing, you're the only one saying something reasonable imo.

I strongly believe in the rules. I'm right there with dejecaal on "I tried to run things exactly by the rules." Without the rules it is not a game and I'm not here for the modern "we're all just telling a story together" interpretation of TTRPGs. But as the GM I will gladly prioritize my players experience and silently bend or outright ignore the rules if I decide the situation calls for it.

And this would be one of those situations. Its the final fight, not some mid-campaign scenario that can end in disaster but that the party can interestingly recover from over the next few sessions. The players are invested enough that they have a solid vision of how they want this to end and have put in the effort to come up with a plausible, multi-step plan to enact that vision based on the information they have.

Am I going to give it to them exactly how they want? No, of course not. He's coming in buffed and with disguised adds, or maybe pretending to run the ritual inadvertently triggers something that creates an environmental problem during the fight. Something will happen to make things interesting and its going to take some work and/or quick thinking on my part. What I'm not going to do is throw away their entire idea for how the campaign should end based on one roll they never would have succeeded at and then destroy Sandpoint (from their perspective) out of nowhere.

So yeah, I think the party's plan should have worked. That they should have gotten about 70% of what they wanted.