So, point one:
Death will lose to life! All curses will be lifted by exorcism, and the little vampires will break their teeth on the armor of the crusaders. The Death mage will cast a spell once or twice, and then he's done for by the angels. Also, a knight hero, having mastered the magic of life from the seminary, will destroy the undead with a holy word. THAT'S IT.
So, point two:
Against Chaos, it's enough to get a ward and build up the angels quickly. This is realistic. And then, if you don't mess up in battle, you can easily handle Chaos.
So, point three:
Against Order, a ward is also enough. And clones deal 50% less damage. Genies don't have infinite mana.
And finally:
Nature needs to be outpaced in development, which isn't difficult. Also, get a ward, and that's it. Crusaders will easily chop up phoenixes. Tested.

2 Optimal
And although I'm not a big expert on 4 heroes, some of your arguments are clearly incorrect.
1. You keep emphasizing the availability of a mass ward against the corresponding faction. The spell itself is undoubtedly powerful, but tell me, how many spells are there at level 4 of Life Magic? If I'm not mistaken, there are 7. And how many spells does the guild give? And that's where you mentioned exorcism (mass), which would also be very good to have. So, the probability of getting a "gentleman's set" on the map without a large number of native castles and shrines is negligible. And if there's no exorcism at all, then it's over.
2. A Life mage, if he doesn't cast spells on himself, will live no longer than a Death mage, but for the same amount of time, 3 turns. Just like any mage.
3. You say that the crusaders already have a ward against Death? But the devils have a ward against Life, and that's often better. Plus, the devils themselves are good as hero killers, and ice demons are used for freezing, for example, angels.
4. Genies don't have infinite mana, but 8 casts in a real battle are more than enough, especially since all sorts of nasty spells are cast on individual genies. And in general, Order is cool.
5. Crusaders will only chop up phoenixes with a large numerical superiority, which may not be the case, given the summoning.
6. Against Chaos, which doesn't have its own dispel, developing the Tactics branch and having a few buffs looks very tempting. I agree with that. But their units are good, a sneaky thief is in reserve, and direct damage, ignoring magic, can potentially be very high against Life's high defense.

Of course, Life has its big advantages: stables in the castle, angels (and sometimes even champions that don't require rare resources), resurrection after battle, etc. If you add a decent amount of spellcasting to that, it becomes sad for its enemies. But this spellcasting rarely appears in full. And other factions also have their counterarguments.