2 Optimal
And although I'm not a big expert on 4 heroes, some of your arguments are clearly incorrect.
1. You constantly emphasize the presence of a mass protection spell against a specific race. The spell itself is undoubtedly powerful, but tell me, how many spells are there at level 4 of the Magic of Life? If I'm not mistaken, there are 7. And how many spells does the guild provide? And there you'll also find the exorcism (mass) spell you mentioned, which would also be very useful. 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 is no exorcism at all, then it's over.
2. A Mage of Life, if he doesn't cast spells on himself, will not live longer than a Mage of Death, but only 3 turns, just like any other mage.
3. You say that the Crusaders already have protection against Death? But the Devils have protection against Life, and that is often even better. Plus, the Devils themselves are good as hero killers, and Ice Demons can be used to freeze, for example, Angels.
4. Djinns don't have infinite mana, but 8 casts in a real battle are more than enough, especially since they cast various nasty spells on individual Djinns. And in general, Order is cool.
5. Crusaders will only defeat Phoenixes with a large numerical advantage, which may not be the case, considering summoning.
6. Against Chaos, which doesn't have its own dispel, developing the Tactics branch and having several buffs looks very tempting. I agree with that. But their units are good, they have a sneaky thief in reserve, and their direct damage, ignoring magic, can potentially be very high against Life's high defense.

Of course, Life has its own 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 number of casts to that, it becomes sad for its enemies. But this number of casts rarely appears in its full volume. And other races also have their own counterarguments.