I'd lose the AT6+ for simplicity.
Here's my version:
Code Sample:
Brass Scorpion
Type     Speed  Armour Close Combat Firefight
War Engine  25cm   4+   3+      4+
Weapon        Range      Firepower    Notes
Scorpion Cannon    30cm      2 x AP3+/AT5+  -
2 x Hellmaw Cannon  15cm      AP4+       Ignore Cover
Demolisher Cannon   30cm      AP3+/AT4+    Ignore Cover
Claws         (base contact) Assault Weapons Macro-weapon, Extra Attacks (+1)
Damage Capacity 2. Critical Hit Effect: The Brass Scorpions reactor explodes. The Brass Scorpion is destroyed and any units within 5cm of the model suffer a hit on a D6 roll of 4+.
Notes: Invulnerable Save, Fearless, Walker.
Let's see if we can cobble an agreed version together in order to have the same rules in both lists.
As you can see, I went for 25cm movement instead of 20+infiltrator. I'm not enormously bothered which way we go on this one.
I went with CC3+, because it's Khorne! It just doesn't seem right for a Khorne Daemon engine to have CC4+. It's very fragile, so it needs to be worthwhile throwing it forwards since it'll likely die in the process.
I didn't go for
ignore cover firefight, because of the issues with the allocation. These should hopefully be resolved by the current rules review, so it could happily go back on once that's decided.
I don't like your idea of always causing a critical when dying, that's not warranted imho, but I did make the critical more deadly to represent the explosive nature of the scorpion.