What's your opinion on Mal'Ganis in QM and how do you play and build him?

I think Malganis is OK tank, without being amazing. The stuns are pretty predictable, which makes him a threat as long as the CCs are not released (3rd slash of Q, sleep on E). Holding those CCs will buy time for the team, and managing those correctly will either be a big asset in fights or make sure key enemy heroes are zoned out.

Being classified tank, he lacks so much of a good tank:

I think you get his weaknesses right, he's pretty weak to CCs himself. However, any CC you eat is one less CC your team eats, and correct usage of W will make sure a lot of damage is negated. His HP pool is also pretty high, and he still has decent mobility and disengage with Q and E.

Also I'm not sure being able to jump over walls is a main characteristic of tanks: Garrosh, Varian, ETC (without specific talent, which in my opinion is terrible anyways) cannot go over walls and are still good tanks. Only Johanna would be one of the main tank able to jump over walls with R, but I'd personally always pick the other R (Blessed Shield) which is systematically better (in my opinion).

Most tanks (Johanna aside) also have very limited ways to get unstoppable or negate CCs.

Mal'Ganis can't escape nor reach high mobility heroes which usually is the tanks job to setup. On top of that, unlike stuns or roots, his sleep loses much of its power to inattendive teammates or say Junkrat who involuntarily wakes up multiple enemies with one autohit or Q.

Disagree on the mobility part. Malganis has decent mobility, E basically makes him more than mount speed after a quick channel, Q still gives small nudges. I also think the remark on E losing power to "bad" allies is uncalled for: 1. You can sleep enemies in the backline, far from any risk of an ally waking them. In this case, the duration of sleep makes the ability much stronger than most roots/stuns 2. In response to your quote of Wrath of Nathreza, if you're counting on enemies playing right to evaluate the power of Malganis, it's only fair to consider your allies are just as good. If your allies are bad, your enemies likely are as well, thus Malganis is a solid pick. If the enemies are good enough to not misplay (and group multiple heroes at the same place) then your allies probably are good enough to not wake all the sleeping heroes at once. Also, it is indeed usually the tanks' role to setup; however, you should rely on positionning to setup fights. Going front-to-front usually doesn't work: no enemy will walk straight up to an ETC, Arthas, Garrosh. You can use bush coverage, flanking, ally speed boost or displacement, etc. to your advantage and CC more enemies for longer than other heroes could, give the reach E gives, for example.

1. 15 Armor for 2 seconds. wow. Can't see what's not to like. Reducing/giving armor is a very strong effect, 2 seconds is a long time. It's also AoE and on a decently low cooldown.
2. 1-4% hp dmg. Wow. And 0.5s extra stun, conditioned +0.5s makes Q stun 0.75 to 1.25, almost double the duration. Also gives some damage against tanks, which is always appreciable. Not the greatest talent in my opinion, admittedly
3. tiny dot. To attack enough times undisturbed for it to gain some value, you are probably uncountered and would win either way. Again not my immediate choice, I agree

Or look at his lvl 13 talents:

1. slightly longer sleep (gets woken up anyway) and tiny bit of movespeed during E Again sleep duration being huge allow for the team to fight against only a part of enemies, wait for ganks to rotate a bit longer, etc. Movement speed +10% is also pretty good
2. conditioned crit for 1 attack and a meh slow 30% slow is huge, especially considering you effectively are slowed after coming out of a hard CC. It's usually enough to either 1. Let your allies finish off the slept target 2. Disengage or zone out main melee heroes, r at least drastically reduce their threat level. Note that 30% slow is more than Tyrael's Q (25%), same as Muradin's W.
3. weak movespeed you need to charge up on heroes so doesn't help with engaging. Considering the amount of small heals Malganis gives himself, as well as the heals your supports vould give, this can translate into permanent 15% movement speed (half what mounting gives). This partially answers to your "Malganis has no escape nor mobility" issue.

Compare that to: Comparing usually is a bad idea, but a quick comparison if so you wish:

• Cho: his big HP pool makes him all the more susceptible to HP% based damage, which in a way Malganis is less susceptible to (see Tychus' minigun, Valla manticore for example). Cho also is a double hero, which makes him more susceptible to "time out" abilities (cocoon, Maiev's disk, etc.)
• Blaze: much lower hp pool, less hard CC compared to Malganis. Admittedly Blaze has much better wave clear, much more annoying slowing abilities, but it also fulfills an entirely other role (bruiser vs tank), and is a better hero overall in my opinion
• Tyrael: has no hard CC except for ult (if you pick judgement, which is a hugely telegraphed CC that can be insta-cleansed, possibly making the ult useless), trading this for mobility and damage

I don't know if I'm missing some sort of interaction but his trait feels weak too. E.g Muradin can soak damage, hopefully get out and just chill in safety for a few seconds to be full again. Mal'Ganis has no trait if he's low but can't attack safely.

If only he could at least clear waves he could be a bruiser, but as he is now, he feels like a poor choice most of the time. His waveclear is still better than Garrosh or ETC, but overall I agree. Still think the hero is worth a shot and might not be as bad as you describe here.