Howlers are in Mordenkainen Presents: Monsters of the Multiverse! Weird that they aren't included here, but I'm sure they could be included in the adventure.
I hadn't noticed this at first, but I love that idea! Pandemonium could really use fiendish inhabitants, and I'm excited to see that Plane fleshed out more!
They appeared in that Minsc and Boo's Journal of Villainy Extra Life supplement, but I don't think that would keep WoTC from putting them in this. After all, Minsc and Boo's had demodands, and that didn't prevent them from appearing in Planescape 5e.
I hope so! I can also see the marketing team needing cool-looking filler art, and just using something they already have on hand, too. I hope you are correct, however! Would be nice to see!