Is open theism allowed?

[Kreuzritter]

Here's the thing, if one wished the argue that God could "foreknow" His own action, but not that of others: If such were true, then the Immaculate Conception still could not be true since the Cross was the mechanism by which God willed the process of Jesus' merits to be realized. Since the Cross involved the freely willed, though foreknown, actions of others, viz. sins, God must necessarily know such things "before"hand within time, but indeed, always, does, and will know them due to transcending time.

This is a good point. God couldn't even have known that Jesus, in his human will, would have achieved his plan. Never mind this makes of prophecy a lie.

[Kreuzritter]

There's different varieties of open theism. The most attractive one says future contingents involving free choices are all false (or neither true nor false).

This is just a re-working of "open futurism" applied, somehow, only to "free choices", the former which just boils down to a denial of the "existence" of the future in any sense, in other words, in this case a denial of frame of reference from which a transcendent being has access to every point in spacetime. As expected. As I said earlier, "open theism" contradicts God's transcendence and is therefore incompatible with theism  as such, let alone Catholicism. We don't even need to get into the question of why its proponents regards a statement like "Gary will choose to play chess tomorrow" to be "always false", "neither true nor false" (if one denies the future, the former statement becomes like "The kign of France is bald", and the former two views are Russelian and Aristotelian respectively) or "unknowable".

What they all have in common is that God doesn't know in advance what free creatures will choose.

Category error. God, as God, doesn't "know in advance". God is transcendent. That you would use such terminology says a lot.

[Vetus Ordo]

What they all have in common is that God doesn't know in advance what free creatures will choose.

This is logically impossible because God is the creator and sustainer of all events, past and future.

There is nothing He doesn't know.