I'll let others reply to your question outlined in the initial post. I'd like to focus on this bit here. Is there a specific reason you didn't either say yes here or allow the PC to roll the dice and make p42 improv effort to weld the door shut with the fire spell? That seems to be precisely the type of thing you want to be encouraging. It fits the genre and it's quick thinking by your player. And it's fun!
If you felt it needed to be hard, all you needed do was (a) make the of-level DC for the Arcana check high. If he passes, (b) let him roll attack vs Fort vs standard, of-level NAD and boom, the metal hinges of the door are slag and it's impassable (save ends)
I recommend that approach in future circumstances.
Sounds like good advice to me.
When you are looking at a TPK and your players are trying to be creative to get out of it, try to say yes to smart ideas. To me, that's one of the big differences between a computer rpg and a pen-and-paper rpg.
Sure it's a bit far-fetched, but you could just as well have said that the doors won't weld shut, but might get hot enough to make the whatever-nots stand back for 10-20 seconds to think about how they want to open the door. Or 2-4 rounds in normal-speak.
I regularily throw encounters like you just described at my players and they usually start to get all creative, looking outside the square box of the rules to solve the problem. Just the thing I want to encourage.
For instance, in one encounter a Knight is challenging a PC and calling him an imposter and going all high-and-mighty. The PC throws off his cloak, displaying a magnificent armor, telling him that it's the Knight who is the imposter. Just for role-playing kicks, nothing more.
Later in the fight, the same PC has killed the knight, but alone, down at 1hp and up against two remaining bad-guys. One loyal retainer of the now dead knight, and one mercenary. The PC offers the mercenary a deal, and the I thought the PC had done a good job earlier to sow doubt, so I allowed him an moderate Diplomacy check, which he aced. The loyal retainer didn't stand a chance.
Anyway back to your situation. Depending on how attached the players are to the characters, you should either just do a full TPK, or have them captured and then rescued by the new PC. I think the last part would be the most fun. Just get the Warlord down fast before anyone bleeds out.
