I'm not sure about how to make their searches advance the plot, and yet limit them as they are not a high enough level to survive that chapter yet. The Cagewrights know that the Temple of Wee Jas was attacked, and should know that Karran-Kural has been taken. Things would start to move with little to no downtime. Lords of Oblivion should begin quickly. It is not a railroading if the NPCs start reacting intelligently. Attacking people that the PCs care about (Striders), and trying to get organized (the party) makes sense. Mix in some politics too, where the Town Council starts moving against the party, especially if they are unable to substantiate why the attack on the Temple was required.
I guess what my ploy here is for distraction. The immediate threat is seemingly more important than the possible future threat. Once LoO is done, this rolls right into FoF.
I may have the same problem. The Psion in the party managed to rip some memories from Ike Iverson before he died and I accidentally dropped the name Fiery Sanctum. So I had to worry about them going there right away. Maybe useing the soulpillers to figure out its location.
Looking into it, I think that the party can only find the Fiery Sanctum from the clues left in house Vhalantru. The area is shielded so that divination to detect its location will fail. Clues left at House Vhalantru will let them know the general area to start searching.
It is the DM's constant delemia - who do we direct the party without appearing to? We all like to give the players choice and create adventures that are based on their interests but in the end a certain amount of railroading must occur in any planned campaign.
I can understand your concern here nib because you don't want to squash the parties motivation but you also don't want to put them in a situation which jumps a great many steps in the Path and also will more than likely wipe them out.
If they are hunting down the Firey Sanctum then I would give them clues but no real answers. Have them find out the historical significance of the location -
One suggestion could be while researching in the Blue Crator Acedemy could be -
The Firey Sanctuum was once the Home of an ancient temple of demonic madness (Adimarchus?) and location of a dormant gate to a dark prison plane where it is suspected that the Demodands first appeared from Canceri before spreading across the frontier lands and creating the Shackleborn.
The demodands and temple have long been destroyed and all entrances to the hellish place sealed. The historical document has a strange sketch of the mining village that would eventually become Cauldron and the entrance that was sealed roughly marked as an open fissure.
The party may then decide to try and locate the sealed entrance within the town using town records but when they attempt to access the documents they are told they are in the hands of the new Lord Mayor - Vhalantru.
Asking Lord Vhalantru meets with a flat refusal with the Mayor stating that all such records have been secured away for the saftey of the city. He won't elaborate any further.
A rough estimate of the location perhaps using a skill (Architecture, Survival, Engineering, etc) would reveal the rough vacinity of the ancient entrance which has been built upon. One of the possible locations includes the manor of one Lord Mayor Vhalantru.
Anyway after this point you need to distract as has already been mentioned with kidnappings etc. When the party eventually descend beneath Vhalantru's manor they find the entrance but it remains sealed. Before the party can find out how to open it, the events of Foundations of Fire occur conventiently breaking the seal with a combination of violent earth movement, larva and the dark magic that is unleashed from the Tree of Shackled Souls.
Anyway this is just some suggestions
Delvesdeep