So because I allowed the players to make a deal with a powerful ally and then had the world react accordingly to their actions, I'm a cupcake DM?
The players entered the throne room with the Blackskull as their escort. As the audience began, Themberchaud began his assault on the Hold of the Deepking. The party convinced Blackskull that they would guard the Deepking and that she should take all other available guards to fight off the dragon. It's not MY fault that RNGesus decided to bestow his favor upon the party, allowing them to make a DC 25 Charisma(Persuasion) check. Had they failed, then they wouldn't have been able to attack the Deepking without guard intervention.
As for the Wall of Force, it's not described in the adventure which shape of wall it was. Therefore, I had to make a field decision of which version best fits the physical description of the room. The wall of flat panels fit better than the dome. Once the party figured out the dimensions of the wall, it was all up to whatever RNGesus decided was going to happen on both sides of the screen.
RNGesus also decided that the Deepking's indefinite madness (which the adventure explicitly called for a roll on the DMG table) was that he doesn't care about what's going on around him. His apathy is what allowed the party to get the jump on him.
You forget that this is a game where DICE determine the outcomes of actions. The DM cannot be held accountable for what the dice say happens. I ran this encounter as close to the letter as possible. The dice decided that the players' impossible actions were going to happen and that the enemies were going to be ineffective.
Sorry that my table's experience ran contrary to what you want to happen at your tables. No need to immediately fault and insult the DMs when things don't go the way YOU want them to.
Expect Table Variation...