Heroes 5.5 WarGame Edition - Tactical Edition.

This is a criticism of any skill tree.
Specifically, in your case:
The Devil's Strike branch in the attack skills. Weakness competes with Stun, and you can upgrade both (different types, but from the same line).

What's wrong with being able to upgrade both? I believe that a choice is always better than a linear progression (it offers more options), except in cases where one perk is clearly superior to the other or directly builds upon the previous one. You are not obligated to upgrade both options; you can choose either one.

And why isn't Fallen placed after Lord of Pain (which is more dependent on spell power)?

Because this is WGE, not the original game. Curse (with a duration of Spell Power/4) also depends heavily on spell power. And Weakness (Spell Power/2) also requires some. These are not debuffs with a duration of Spell Power/1, where spell power is almost not required.

Readiness and Solid Defense. Here, Readiness depends on Solid Defense, as one enhances the other. And Readiness deserves a level 3 upgrade.

I disagree. Readiness is not needed for mages whose task is simply to survive in defense, not to fight back. Furthermore, I doubt that Readiness deserves a level 3 upgrade – Solid Defense looks more appealing (on average, it provides 5+ extra defense, which, by the standards of the original game, is 10+, and it retains the bonus when waiting). I suspect that Solid Defense might even need to be nerfed.
The choice for Readiness allows you to upgrade either Solid Defense without Readiness, or Readiness without Solid Defense. This is already 2 options against one. And I would like to see this perk (Readiness) being chosen more often than usual. And usually, it is never taken (because the power of the perk does not match the level and secondary requirements).