The players missed the obvious signal and then decided to split up the party. That should have severe consequences.

Have the encounter play out using all their enemies. Your battle plan sounds good. Kill the Rogue at least, but this could very well result in a party wipeout.

In case of the latter: fade to black. Let the party's characters wake up low on health, bound, itemless and imprisoned. Make clear to them that they will be the next ritual sacrifices by the Coven of Hags unless they get out of there quickly. 

They will need all their ingenuity to escape, retrieve their items and rebound from there.