A great read and a masterly guide. Like you, I am a 30 year veteran of HQ, Xmas 1989 I broke the seal and never looked back, on my third generation of players now.
However, you can't post something with a name like "Ultimate Guide to HeroQuest" on a forum like this without expecting some challenges, so prepare yourself for the onslaught...
p42 - Hero DeathIf a hero is reduced to 0 Body Points and they have a healing potion in their possession, they may drink it immediately to avoid death. Likewise, if they can cast spells and have a healing spell, they may cast it upon themselves assuming that they have not already taken a standard action on their turn.
Reading this implies to me that a spell casting Hero with a healing spell can use it to avoid death, only if it is their turn and they haven’t yet used their action. But death caused by what? Any monsters (and indeed other Heroes) cannot attack if it isn’t their turn, wandering monsters only attack the person who conjured them as a result of a search (an action), and the rules on traps state that if a hero suffers an injury from a trap or hazard, then their turn ends. If I'm not alone in struggling to conceive of a situation in which this rule could ever be invoked then perhaps dropping the rule might be simpler.
p49 Diagonal MovementNote that neither heroes or monsters are obliged to move their full allowance, or at all, and they can never move diagonally.
It may be that a hero desires to jump over a particular square on the game board, almost certainly because there is a trap on that square.
A hero must be adjacent to the square that they wish to jump, but they are not limited to jumping to a safe square two squares ahead of them—they may jump to any other square that is adjacent to the trap, effectively allowing them to jump diagonally from their position.
Your use of the terms "desire", "wish" and "almost certainly" implies to me that Heroes can choose to jump a square whenever they get the urge (and nothing in the rules states that isn't the case), for example choosing to jump a square directly in a doorway into a room to avoid any potential nasty surprise but combining that with the ability to jump diagonally seems to negate the never move diagonally rule.
What would be lost by scrapping the no diagonal movement rule, HQ is all about simplicity and one less rule must be simpler indeed?
p57 SearchesSearch for Treasure
As normal, that hero cannot search the room again, though their companions may
Search for Secrets
Each room and corridor in a dungeon can be searched only once in total, for it is assumed that the first hero to search them will have found all that there is to find.
Search for Traps
Each room and corridor in a dungeon can be searched only once in total, for it is assumed that the first hero to search them will have found all that there is to find.
May just be me but that positively oozes contradictions, if you are searching an area for secrets or traps then
obviously once it has been searched by a Hero once then all that there is to find will have been found, but search for treasure and a whole different mindset is assumed. bearing in mind that Heroes maxing out on equipment early on is a recognised issue with HQ then extending the one search per room logic to include treasure seems a reasonable improvement.
P59 Furniture TrapsNote that, in the case of furniture that contains traps, it has never been officially stated that a hero must move adjacent to that piece of furniture in order to disarm it, but I would argue that this should indeed be the case.
A Hero can search a room for treasure, finding gold in a chest on the other side of the room and collect that gold without the hero needing to move adjacent to the chest, but if that chest was trapped the model would have to be moved adjacent to disarm the trap?
p62The heroes are able to sell unwanted equipment between each quest for half of its normal value rounded down.
However on the previous page under the potions section, you mention that whilst many players allow purchasing of potions between quests this is not mentioned in the base rule book but only in some of the expansion packs so you conclude that this rule applies only when you are playing the relevant expansion pack. It is my understand that the rule around selling unwanted equipment also only appears in certain supplements and not in the base rule book (although that might be EU rules rather than NA rules so would the same logic not apply?
And that was just the first read through, I will return my attentions post mince pies and port