Greetings of Peace. This subject is given ample treatment in Schuon's article "Creation as a Divine Quality" published in Survey of Metaphysics and Esoterism. To identify just a few salient points as they pertain to your question we might say that the necessity of manifestation is explained through a tautology metaphysically considered, which is to say that it rests upon the certitude of Divine qualities apprehended through intellection as opposed to ratiocination.
Specifically, manifestation is a necessary corollary to the quality of Divine Goodness whereby it is in the nature of the Good to radiate itself. The hierarchy of the multiple states of being extend in the direction of the negation of the Principle through its quality of Infinitude. As Zachary once eloquently stated - if I remember correctly, the Absolute being without limitations and restrictions is also beyond the state of being without limitations and restrictions.
The preceding discussion has been limited to the consideration of manifestation only. In his article, Schuon makes the distinction between manifestation as such and creation. The former pertains to total manifestation which proceeds of necessity from the Divine nature, while the latter pertains to partial manifestation or a single cycle of total manifestation which proceeds freely though not of necessity from the Divine Will. In his own words,
"As we have just indicated, cosmic and coeternal manifestation is necessary, because God is necessary, whereas "creation" is free because it is, not "the manifestation," but "a manifestation"; God is in fact free in His "modes of expression," but not in His "ways of being," so to speak, and this is necessarily the case since the perfection of freedom and the perfection of necessity must both be found in the divine Nature; despite the anthropomorphist theologians who see a constraint in necessity, and who unconsciously confuse freedom with arbitrariness, hence with the absence of principles.
Indeed, God was not obliged to create "this" world, that is to say, these minerals, these plants, these animals; but by His very Nature He was obliged to create "the world as such": by His very Nature, that is, by virtue of the Radiation demanded by His being the "Good.""
I realize that I have not answered your first question regarding the details of evolution which is beyond the scope of my competence and interest. Undoubtedly, there are other participants here much more learned than myself in this domain who may be able to provide you with adequate feedback.
Greetings of Peace. This blog contains selections from my correspondence and other sources on a variety of subjects related to religion, philosophy, and spirituality. I hope that they may be of benefit to the interested reader. Concerning the title of the blog, read this entry.