What makes a shield wall impossible other than you saying it is? They are something that existed in the real world, but D&D doesn't model military unit tactics. So I improvised. It's a very situational tactic, if there were a similar situation for the PCs they would be free to use it.And you again fail to point out what getting close with the cover would actually accomplish. By now its obvious that you have no idea yourself.
You have only given the PCs another tool to use by ruling you can craft mobile cover which is effective against ranged attacks and spells for free. The next dungeon will be fun when the PCs use such a tree shield to freely move around.
And you are completely ignoring the vision issue.
But the highlight is that now people are carring around gallons of paint in throwable containers for the random chance to encounter a tiny hut. Not that they would be of any use as the paint is easily scraped off because of the hut. If paint would even stick to a wall of force which as the name says consist out of force and not matter. Thats like trying to paint a magnetic field or the sound barrier.
He actually never said that. He only admitted that they screwed up when writing the spell but never said that it should be changed.
As far as what the hobgoblins were going to do, they never got that far. Plan was to block line of sight for one direction then start digging. Well, bring in the half dragon half giant lizard mount* and have it dig. Once there was a hole, breath fire.
Unless of course the PCs swarmed out to counter attack, which they did.
The gallons of paint could have been used back at the castle. More likely a few gallons of some type of sticky flammable substance that would block line of sight and cover people when they left the hut or the spell expired. Have some guards at the ready with some sort of fire based attack to take advantage that the PCs are covered in pitch.
*No I didn't mention the half-dragon half-giant lizard mount before. There's a great deal about the scenario that wasn't particularly important. Half dragons were a big part of this arc of the campaign.