How do I move forward when an impending doom was stopped by accident?

I’m having an issue in a campaign I am running where an impending doom that was part of a front was stopped by the players pretty much by dumb luck.

In order to both prevent any of my nosy players from getting behind the front info, and to make this simpler I will use a made up example of the same issue I am facing:

My front has the impending doom:

The Dark Serpent DeorcaĂžexe is awoken by King Cyndemund’s meddling.

And I have an early portent:

King Cyndemund’s scouts retrieve the tomes of arcana.

This portent comes to pass, and the players meet a suspicious pair of travelers staying at the same inn. The pair some sort of box that they are heavily guarding. The players reason that although they look suspicious, many people look suspicious and it just isn’t very important so they will ignore them.

Now this is fine, ignoring my front is something I can totally deal with. This is certainly not the first time this has happened.

However short while later when the party is a bit strapped for cash they decide that they remember these suspicious travelers and figure that they must have something valuable. So they attempt to steal the box, things go south and they end up murdering the travelers. They find that the box contains some boring old books not gemstones and burn the evidence. The party wizard uses charm person to pin the crime on an old man also staying in the inn. They use the reward money to pay off their debts and the party leaves the town later that week.

So now with the tomes destroyed my front obviously has to change, but I am a bit conflicted. Usually when the party ignores a front I want to increase the danger. As far as the party seems to think these travelers were unimportant and I was being stingy with distributing loot. I could make another way for King Cyndemund to access the magic he desires. However I also don’t want to railroad anything, we are playing to find out, and "Oh you destroyed the tomes, but it turns out there was a second copy!" is not really in line with my principles.

So what is the right path here? Do I consider the front resolved even though the players ignored the portents? Do I repair the front and ignore the players actions because they were incidental?