On "Modal Collapse": How Can the Creative Act Be Free (If God's Essence is Simple)?
Because His Essence is being-itself: the exemplar of any possible configuration of [created beings,] means to [the extrinsic manifestation of] this essence. It/He is not diversified or added-to by the free creative act (or any of its permutations) as if by extrinsic difference[s].
The creation instead fractionally participates in what God is comprehensively, with its variability due to the inexhaustible & infinitely diverse ways His Essence can be participated, not some alteration in this essence.
Further: what does it mean that God contains all things & how does this inhibit modal collapse?
There is modal collapse only if being is univocal: if God's simplicity is one thing that would somehow be diversified by the free creative act. In this scenario, God's simple essence would be either-or, the same way that I, as a finite being, am either in place a or place b but not both simultaneously.
But being is analogous (remember we know this early, as a condition of "resolving parmenidean monism", of there being many, limited, changing things). It is already internally diverse. This is especially true of God, the exemplar of all being. Such is the answer Maritain gives to the question of how "the Word can communicate His Divine Existence to the human nature of Christ", which applies equally to the question of how His creative act can be free or superfluous or not-necessary: He already "eminently is all things", there is not "more being" after creation, God Has nothing to gain, He plays in making the world.
God is, in other words, not diversified by the free creative act. His essence is "beyond one and many, absolute and relative". So He is unchanged if He decides to create or not. His simple essence extends to all and any permutation of possibilities. This can be so given the analogicity of being & God's eminent containment of all things. Only a problem if He & being are univocal.
If we consider God’s reason for creating (instead of His creative act):
"God's reason is identical to his essence, so the reason for the existence of a contingent thing, or combination of them, is actually something necessary" appears to be a variant of the modal collapse argument ("if God is simple, then the creative act is eternal/divine/etc").
This argument seems to ignore the asymmetrical nature of potency-act relations; potency participates or shares in what actuality already is. So it seems to confuse God's action (its final causality), identical to His essence, with creatures or participants in it that are only secondarily its object.
The creative act is free in whole or part because it is gratuitous. God creates for the glory of His essence. But whether or not He does, He has nothing to gain, is the same, already fully glorified. This is a straightforward consequence of His status as pure act, first cause (and everything else as participants in His actuality). We might say "God is equally compatible as is with creating or not creating".
Exactly because His action & reason for it are identical to His essence, they only touch on creatures inasmuch as God already precontains them in His essence; likewise, His creative action only manifests (ad extra) what He already is. Just like in thinking of a creature He is only thinking of Himself, in creation nothing is added to Him.
So it is "necessary" God love Himself, necessary His action is the same as His intellect (& essence, will etc). But it is not necessary to God's love of Himself, nor His eternal pure activity for this end, that His action have any effect "outside" Himself. God isn't irrational for choosing any combination of finite things or indeed none because they are gratuitous & none are more necessary to Him (the end of His own glory) than any other. All creation is unnecessary or instrumental to His reason for creating.
So we see why there can be no "best possible creation" or necessary reason for one or another configurations of creation. There can be no determinate relation between finite things & God, perfection itself. No finite thing better serves or contributes to God's already fully actual glory. Every finite thing & arrangement thereof will always be unnecessary, partial, admit of further perfection & always fall short the standard (as distinct from God, to whom they are gratuitous). This (again) because potency participates in act (as creation in God). A "best possible world" is contradictory in metaphysical terms, like the notion of a "pure idea", or "uncaused contingent thing" [something not intrinsically actual but neither extrinsically actual].
It’s also worth noting what freedom means in an analogical sense; to say an intellect is free regarding something is to say "there is no necessary connection between it and this finite good". So WE are free regarding the choice of particular goods with no necessary relation to our last end; our choices of food, music, occupation, etc.
In light of the above, it is clear that God is by definition free regarding anything outside Himself. Since he is act to their potency, there is no necessary relation of Him to creatures; He already eminently IS everything any creature could be.