You give a lot of things to think about but on the Wizard, we can respectfully disagree.
To start off with, magic is awesome. Several spells work instantly with no enemy defense (before the effect), or work instantly to help the hero (even saving him from death). Also spells are ranged, which in the early quests, is very unique, in that the only other ranged attacks (in the NA version) are thrown daggers and the (expensive, not usable by wizard) crossbow. Forget about passing potions, when it come to healing, the Wizard (like the Elf) can do it at a distance too, but only on his turn. Unlike the Elf he gets NINE spells (and first pick at that!) and they recharge between quests at no cost. The difficulty in playing the Wizard is knowing when to use his unique skills.
Depending upon what spells he chooses (but more importantly how he uses them), the Wizard is an incredibly valuable character. Many people assume he's only got three attacks... the obvious Fire of Wrath (1 BP), Ball of Flame (1-2 BP) from Fire... and Genie (1-5 BP) from Air. Most monsters have only 1 body point, even in the "harder" NA version. But it goes beyond that.
Sleep is a very powerful spell. If the Wizard chooses Sleep, there is a very good chance the other Heroes can quickly kill any "boss" monster in short order (or, depending upon Zargon's luck, the Wizard himself can do it on his next turn with one of his big attacks).
In later quests you get some very powerful monsters that have very low mind points (Giant Wolves and Ogres come to mind), making them sitting ducks for this spell. Let's sit and consider for a minute just how awesome Sleep actually is...
In the game system only the Goblin has 1 mind point and only the Orc has 2 (so why waste it on a monster with only 1 BP), but look at the other quest packs:
Ogre Warrior
Ogre Champion
Giant wolf
... each have multiple body points, but only 1 mind point! (and that's true in the NA conversions as well)
Other monsters with low mind points:
Ogre Chieftan: 2 mind points
Enemy Mercs and Dark Warriors: 2 mind points
Polar Warbear: 2 mind points
Yeti: 2 mind points
Elven Warrior: 2 mind points
Elven Archer: 2 mind points
the mighty 10 body point Ogre: 2 mind points !
And yet it's also quite possible to sleep a Gargoyle (with 4 mind points)... all based on the dice.
Courage is a very useful spell to "buff" himself or any Hero he can see, but according to the NA rules when the monsters are out of sight, it ends. The best way to use the spell is of course in a room! The one concession I make to Courage as written is that once it is active, I allow the one it is cast upon to move back into sight of monsters at the end of his turn to keep it active (technically it says any time he loses sight that's it). And since in another thread I admitted to having too narrow an understanding of "see" it's even more useful.
There's nothing in the rules of HQ that say the Evil Wizard player has to make every monster dumbly attack the nearest hero, even if this is the case in some other types of games. Zargon knows where the Heroes are at all times, and so do his monsters, and everybody knows everybody else's body points (the rules don't explicitly state he has to tell the Hero players the body points of boss monsters, but I do when I GM). Zargon knows all ! Other games and house rules may differ of course.
Tempest causes a monster to lose its turn, which is actually far more useful than it appears. A monster who loses a turn gets to do nothing: he can't attack, move, or cast spells. This allows escape or your guys to get into position to fix his wagon.
The often underused Veil if Mist lets you move past monsters, and Swift Wind doubles movement. Helpful for getting away from monsters and escaping to the stairs or getting to your destination.
Healing-wise, the Wizard is guaranteed to have at least one healing spell, which means his "puny" 4 body points are effectively doubled (consider the Barbarian has 8 BP but no natural healing ability). Yes, if you have only one you might want to use it to save a comrade, but if it's just about self preservation, you're set, even if you argue that some very strong monsters could get a lucky roll and wipe you out in one hit. The EU rules doesn't let you save yourself from death.
Pass Through Rock is a great way to take a sneaky shortcut, or to jump right into the action where you're needed (sure it could also get you in a jam but if you're smart, it's very useful).
Most often the Elf takes the earth spells, but Rock Skin is a very useful spell as others have pointed out, since you can use it at the beginning of the quest and as long as you don't lose a body point, it could last the whole quest! Even one extra defense dice is very helpful (EU version it's super powerful with 2 dice).
In the NA game, sure, the Wizard (by default) lacks the Cloak and Bracers, but he eventually finds the Wizard's Cloak (3 defense dice) and the Wizard's Staff. Sure the staff may be "only" 2 attack dice with a diagonal hit, but it's a far better deal than just having a dagger. I've seen Wizard's get strings of lucky kills too, so it does happen, but many Wizards play conservatively.
It's true that the Wizard is not built to be a tank. The NA HQ is definitely built around the Heroes "working together." Sure, the Barbarian could run off on his own for awhile without help, but he has no healing abilities... he has no way to disarm traps (toolkits are expensive and only 50% effective), and on top of that his mind points are low (against the eventual Chaos sorcerers).
Teamwork doesn't happen automatically but I like how even within a game it starts to come together. As Zargon you're trying to kill the heroes, and hinder them as much as you can (within the rules). The only thing stopping you from being absolutely ruthless all the time is the fear that players will give up and go play fortnite. Some Zargons play such that they only pretend to be dangerous and ensure the Heroes win (or win unless they do something really stupid). This is explicitly how "the Demon King" is ordered (secretly) to play in the Japanese rules... but not the other editions.
So while the (hero) Wizard may not be a monster slaughtering machine, he's also able to keep the other Heroes alive and to help them kill monsters (Or get past them). So instead of thinking about him as "only" being able to mess up four monsters, think of him as making the difference between victory and defeat, or between losing a hero (or two) permanently and having 'em for the next quest.
If the Wizard started out fully buffed out (or as I have him in my upgrade system) nobody would want to use any other characters.
As I believe the Bard humorously put it, the Barbarian is basically "easy mode" when it comes to the Heroes, and there's nothing wrong with that. But even the Wizard got a "canonical" solo quest (granted, he can get help later), so there you go.
PS: One more thing... the Elf can use any of those spells, the only difference is he picks AFTER the wizard, and he only gets the set of three (unless using the aforementioned suggested house rule).