The reason the monster don't open door before they see the PC is because they are not aware that something is happening. But after round 4 all door are open and they are all very aware of something happening, with the dungeon exploding and all.
Technically speaking, shouldn't all the monster in the dungeon be able to converge on the PC now that they don't need to open any doors to do so?
I hadn't interpreted it that way, but I wouldn't object to that interpretation. One thing to keep in mind is that there are few safe paths for many of the monsters in the lair to reach one another after that event occurs. That said, I'm surprised KarinsDad didn't have one of the two temple champions open the back door to their room when everyone bunched up at the portcullis. Losing one round to bring in two more fresh allies seemed like a given.
Likewise, I'm surprised that neither of the warlocks in the mudroom managed to open the door behind them to shout an alarm to their master. That would have let his team complicate things nicely.
As for the complaint about the box telegraphing the importance of fire resistance, that strikes me as entirely appropriate. I mean, wasn't "Forge of the Dawn Titan" a dead giveaway what to expect? If parties prepared in advance for fire damage, good! The metagame is justified quite well within game: wouldn't Lord Neverember have hired people well equipped to tackle fire-based opposition here?
I was surprised that only two of the eight players I ran in simultaneous sessions on Saturday opted for fire resistance--but not as surprised as they were with how that option turned out. (Useful, but not anywhere as useful as they'd thought it would be.) One group got bogged down at the front entrance and refused to move until they'd killed off everything in the room...and while one had exceptional counterattacking ability (doing at least 17 hp of damage to anything that made a melee attack against him), that turned out quite poorly for them. They eventually made it through the sealed door (on their first attempt, as a 20 was rolled on top of an attempt to aid another), and the last man standing managed to survive for another six rounds by virtue of an amusing string of bad rolls (nothing over 3) on my part.
The other group, however, managed to get through those doors and swarm all over the boss by round three. It was looking good for them, with action points sprayed everywhere and a prone and dazed target before them. And then their luck ran dry, leaving them unable to tag him at all as the other monsters came in behind them...the cleric got knocked unconscious and was dumped into the lava with fatal results, the rest of the team found itself immobilized long enough for the enemy to regroup, and they were completely slaughtered within the next two rounds.
I'm already working on alternate trap and monster placement, with a few nasty ideas for the alternate monsters in place. (The Elect can do a surprising amount of harm if they get a chance and you think beyond what they can do with an attack roll.) I would certainly like to see alternate trap and terrain options in further assaults. Easier means of revealing the map a room at a time would also be nice (laminated pages I could fit together instead of a poster of the full map, for example). As an introduction, however, I am quite happy with it, as were at least six of my eight players.