Do casters rule and martials drool?

[Deathtongue]

So wait-a spell that exists, but can't really be dealt with other than dispelling every room, is fine, but a new wrinkle that can be dealt with in another way, possibly leading to a climactic scene where you desperately try to smash the orb to teleport away in time, is bad?

It might be. When a DM has a high-level, reasonably well-prepared foe lock out easy Teleportation with Forbiddance they're just using a setting element. Teleportation lockout isn't exactly something you can do willy-nilly, but it's as basic of a precaution for high-level vaults like high quality locks and an armament for the guards.

The latter, you have to be a lot more careful about. If the DM keeps coming up with reasons (or, more to the point, excuses) why your tools won't work, them hiding behind the logic of genre tropes or in-setting logic is going to fall flat. It's going to feel more like the DM is trying to screw with you, much like if you were playing a rogue that invested a lot in poisons and sneak attack and the next few adventures had the hardcover alteration of having the main antagonists being sentient constructs and undead instead of the humanoids that were originally planned.

How fantastical is your spellcaster if all he takes is attack spells?
How fantastical is your spellcaster if all utility spells he knows have no use (like waterbreathing in a 'dry' campaign)?

You tell me. There are plenty of ways to sandbag your character. There are plenty of ways to build your character in such a way to make you specialized in certain areas (such as social campaigns or blasting) that will close off certain adventuring avenues. So on.

It's a choice casters have to make. Some can recover easily from their choices (clerics) others can't (sorcerers). But note that martials never get to make a choice in the first place. Barbarians don't get to decide whether they want to use their free level-up spells on Demiplane and Teleport instead of something more suited for blasting or diplomacy.

In a world where teleporting is a thing ... how do you think it is sensible there wouldn't be teleportation countermeasures?

Depends on the context and depends on the teleportation countermeasure. Even if Forbiddance wasn't in the 5E D&D, there are still easy ways to deal with people who try to teleport into secure areas; A glyphs of warding or a Symbol tucked away in the vault will pretty much screw over any kind of teleportation.

However, when a DM regularly invents or introduces heretofore-unseen-by-the-rules reasons to screw over teleportation or any kind of player tool, you should start looking for railroad tracks.

Do you think that the Mechane was here because the author went "let's roll on this table what random event will happen" when Elan was separated from the Order and he got "friendly airship captain", or did he decide that for narrative reason, there needed to be someone to let Elan travel in a hurry and teach him how to be a badass, and so introduced someone in the story that would fit that role?

Reductively, the answer to that question is going to be the latter. Because OotS is a comic and not a TTRPG. Even if Burlew was generating his plots with a random number generator, it's still his choice to go through with generated results or not. Not so for a TTRPG, which puts a lot more limits on both players and DMs on ignoring the results of random number generators.

That said, I think my comparison is still valid. Order of the Stick, both in parody and in seriousness, also tries to emulate the tropes of TTRPGs if not D&D. Things happen in the comic in such a way to (usually) have a D&D session analogue, as if you could recreate what happened in the strip with minimal fudging. I'm aware that comparisons of D&D to Order of the Stick are limited by the natures of the mediums, but Burlew does makes a strong effort to make his comic reasonably transparent. And one of the tropes that the comic is built around, which is a trope gaming sessions must IMO be built around is that not having the tools expected of you are going to limit the scopes of your adventures. And the adventures that you still can go on are going to be uniformly harder with no compensation or silver lining. OotS does introduce some ways for the PCs to get around obstacles caused by their poor party build, but it also continually extracts its pound of flesh. And more importantly, the comic also does try to make the alternative paths plausible or at least appropriately foreshadowed.