These modern rules of etiquette are weird...

It's possible that at least one of the team members had watched Knightmare before and knew who he was

Somehow I find that hard to believe after hearing dungeoneers left, right and centre refer to Hordriss as 'an old man'. That indicates a lack not only of knowledge of his name but just how much respect the streak-bearded mage commanded.

I mean, if you're going to call Hordriss an old man, you might as well skip up to him, clap him too hard on the shoulder and ask how his love life is going.

Then get turned into a lizard.

(Of course the game planners would not take that for granted and you're right, there must have been a continuity blip somewhere. Just offering a sensible explanation to the story so you can sleep at night )

Does anyone know whether Merlin revealed what his calling name was? That's assuming he had one...

Giles (Team 8 of Series 4) called out "Merlin" three times to summon him. Yet this seemed to be something of a continuity error: when they'd met on Level 1, Merlin hadn't introduced himself, and in fact couldn't have even if he'd wanted to, due his mind being imprisoned ("I know not even my own name").

That said, Hordriss, the proudest of wizards, didn't seem to have any real problem with it. Perhaps because his nature was that volatile that he knew nobody would dare mis-use it.

He did give other names that he went by, such as the legendary 'Bert', and 'No', but they weren't calling names as far as we can tell.