Sounds like good advice to me.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.
Is having a prepared plot really considered railroading?
"What would you do if you had a million dollars?"
"I'll tell you what I'd do, man: two encounters at the same time, man."
This seems like a natural consequence to me.Take 'em captive, they can meet the new player in the pens, make a daring escape from there.
Like letting Burning Hands weld the hinges together for a few rounds. In my opinion, that was a good idea by the player. Why did he/she do it if you said it wouldn't work? It seems like a great opportunity to reward a player for clever thinking. I encourage that, regardless of whatever else may be happening. I think it is much better to be a "Yes, you can do that" GM than a "No, that doesn't work and here's why" GM.* Also would have let the PC have a Moment of Cool.
Amen to that. It takes experience--aka dozens or hundreds of game sessions--to be able to mostly predict what your players are going to do. And then, they will do something you never expected. That happens all the time. You just need to learn to roll with it.Figuring out your group is half the battle.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.