Personally I'd avoid duplicating the price formula in the room code, just in case it ever changes in the future. That way whoever changes it wouldn't get a nasty surprise when they find Mondedr is still using the old price formula.

Speaking of duplicated code that didn't get updated, we currently have this block to let the player take back quest items that somehow ended up on a shop floor:

  if(Customer->CanBeSeenBy(GetMaster()))
  {
    if(ForSale->IsHeadOfElpuri() || ForSale->IsGoldenEagleShirt()
       || ForSale->IsPetrussNut() || ForSale->IsTheAvatar()
       || ForSale->IsEncryptedScroll())
    {
      ADD_MESSAGE("\"I think it is yours. Take it.\"");
      return true;
    }

For some reason it checks against a list of items rather than ForSale->IsQuestItem(), which means that the Xinroch quest items can't be reclaimed at no cost. The code for selling items makes the same check.
Ivan's gas mask is for some reason a quest item, so if the shop code was changed to use ForSale->IsQuestItem(), the gas mask would become exempt from shop trading.

Like a 0% discount? It would take more or less the same code changes as applying a percentage discount to mirrored items. item::GetTruePrice() would have to either take an optional parameter or be split into two functions so that mirrored item prices can be calculated differently when buying them from the Mondedr shop compared to other situations.

With just a flat markup it would be pretty easy to distinguish the mirrored items from genuine ones - more than one item would have the lowest price in the shop (and items of different value selling for the same minimum price would be highly suspicious).
Another way might be to change the pricing behaviour of mirrored items, so that their price is evaluated differently when being bought from a shop (or that shop). Then the mirror item could be 50% or 90% off (but marked up 300% because Mondedr).

Wouldn't they thank you for clearing mirrorrd junk out of their shop?

Inability would be a bit extreme, even for IVAN - maybe it could act as a multiplier for relation changes with gods (depending on their alignment)? It could even block standing increases from certain sources, so that you can only improve the relation by sacrificing stuff.

Fire resistance is already something players seek almost as a prerequisite for GC/XT - maybe the dragon could breathe acid or poison or something?

Ommel rennet.

OK, I've gotten a victory. Not quite sure whether events happened as intended.
Start-scummed for INT and WIS. After exiting UT with some loot from a bones file and a Mjolak, I went for artificial limbs as soon as I could, severing all my limbs with meteoric steel bear traps and praying for replacements. Got two good limbs and two bad ones due to mistakes and bad luck, but it was enough to win at sumo wrestling.
After a bit more exploration and sci-talking I was able to get phoenix feather limbs and gear. Mustard gas was plentiful enough to kill Petrus with a betrayed dwarf, with a veteran guard and a couple of bottles of acid helping to retrieve the Justifier peacefully.
With this I got to XT10 and killed Xinroch's ghost. Here things got a bit weird. I found Xinroch (the original one) soon after killing the ghost, killed it, and took its ruby flaming sword. I went back to Attnam to see what the necromancer said about all this. He necromancer did not care at all about the "wrong" flaming ruby sword. Is the old Xinroch supposed to spawn while doing this quest?
Anyway, I figured that I had to go through the portal to get the "right" flaming ruby sword. Having playing GC10-13 before I thought it would be wise to beef up first. I had found 4 mirroring charges and wished for a 3-charge cloning wand. Scrolls of charging were plentiful. I didn't get any ommel cerumen until sci-talk was almost finished, but with so many spare cloning charges and good INT/WIS hats it was enough to let me talk to the dolphins and upgrade my gear to GEF/Valpurium.
My normal practice is to bring a horde of wolves to GC6, drink lots of ommel snot, get 2-4 GEF limbs, and kill Izzy in a controlled environment, so that I can be overpowered for the rest of the game without any worries. Absent any empty levels in XT, I brought the wolves to XT2 instead. Upgraded and waited, but the resurrected Xinroch spawned before Izzy, carrying the quest flaming ruby sword. Is the resurrected Xinroch supposed to be allowed to spawn outside the last post-portal level?
Either way, I cleared out of the Izzy-zone and took the sword to the necromancer. He helpfully let me know what to do to get the dark knight victory. I obliged, but I've made a copy of the save file so I can go through the portal and maybe win "properly" if this wasn't the intended sequence of events.

My reflections on XT as an alternative to GC are mostly going to echo JoKe:

GC throws serious danger (wolves) and serious treasure (minefield) at you quickly. Most of TX felt like GC1 for both risk and reward. I still died a lot in TX, partly because of undead spawning ahead of the power curve. In general the handmade features of TX are memorable, but scarcely impact gameplay.

I liked the oasis. The lab was cute but didn't impact gameplay beyond stopping me from praying to Silva. The shop was worth scamming 3000 gold from, but otherwise useless (I did eat all its food, but only because I was running around the dungeon while stressed). The gas chambers always blow up within a few turns, and if I'm lucky I can salvage 3-5 items from the wreckage. The enner kids worked well enough. It might make things more interesting if they're on the same level as the lab, and their screams break the locks on the other cages. The level with the neutral necromancer (I think it's always the lab level) was terrible. Undisplaceable monsters are OK in open areas, but in corridors they're tedious (and leprous). TX9 was good, the spellcasters hidden in doorless rooms were tough to kill. As the level layout becomes more familiar it might get easier to assassinate them quickly without teleport malfunctions.

For TX2-10 I felt both risk and reward were decreased from GC. A lot of the reward of GC comes from the two big chests, the minefield, and the zombie level, which here are replaced by one chest and the gas chambers, which are less than half the size and mostly destroyed before the player can reach it. Apart from the enner and frog levels which it has in common with GC, most of the risks in TX are kept behind glass walls until the player or a freak accident lets them out - the oasis dwarves are behind water, the mutants are in locked cages, the necromancer and zombies are neutral. By comparison, the hazards in GC are unavoidable - the wolf room is almost always unlocked, the minefield needs to be explored as quickly as possible to avoid losing flammable items, the zombies and veteran dwarf on GC6 swarm you given half a chance.

In GC, the succession of challenges facing the player makes for a lot of roadblocks - they need at least one of a selection of stats/abilities/items/equipment before they can attempt many of the levels safely. This often forces a choice between going in unprepared or using resources inefficiently (using wishes and charging scrolls, enchanting non-endgame equipment) to increase survivability in the short term. For most of XT, the only roadblocks are UT4 and the (deferred) enner level. Without that kind of pressure to consume (but also with a lot of luck), this character was able to save all their scrolls of charging for the final equipment/limb boost, despite the reduction in minefield/chest treasure.

Bugfixes aside, future TX development should focus on active threats for the player to face. Weren't there plans for a spider boss? Maybe that would make a good stand-in for the wolf room.

Other random things:
* TX level numbers seemed to jump from TX3 to TX5 in at least one of my games. Edit: Oh. Right.
* The fountains full of glowing blood fill cans and bottles with 100g of fluid, rather than 500g or 1000g, and the message "You don't dare drink from this fountain" doesn't make much sense when I can drink the blood from containers without any harmful effect.
* The necromancer's message when I show him the sword is weird. First he tells me to go and get it, then he says I shouldn't have it because I'm not Infuscor's champion?

OK, I've loaded the backup and done the rest of the dungeon. Squashed Izzy and took a resurrected Sherry along for the ride. The resurrected Xinroch spawned again at the bottom of the portal level, and I ended up with two quest swords. Kind of wondering what would happen if I disarmed Xinroch, left him alive, and tried to claim the victory. Can't really comment on the difficulty of those levels, since I broke the power curve before going in. The minified mage room is nice, though.

I've seen this outside of WM too. It was very mystifying. And bad for my health.
It definitely felt like a bug when I encountered it - I didn't report it on github because I wasn't sure how to reproduce it. Making limbs vital to a ghost seems a bit weird; making them disappear seems reasonable. Redirecting all limb damage to the torso seems a bit strange.
Perhaps there's some way for ghosts to keep all their limbs, but have them excluded from targeting when their hp is 0 or less?

What intensity is used for the menu/victory/defeat tracks? I'm assuming 64, but I can imagine some of them playing while the player's body parts exist and have HP.