Bill, it seems to me that your position does imply determinism assuming that the past and future are symmetrical. That is, do you contend that the future is as real as the past? If so, then events in the future must also be determine. But, the only way for events in the future to be determinate is for some form of determinism to be true. Example: Will Bill drink a glass of red wine on April 15, 2020? If presentism is false, the fact of Bill's drinking or not drinking of a glass of wine in two days is no less real than whether Bill drank a class of wine two days ago. Nothing we can do now can change the fact that Bill did or did not drink a glass of wine two days ago. If time is symmetrical, the same must be true of what will occur on April 15.

I agree with Brian that a presentist must reject (2). But it is not clear to me why a presentist should be hesitant to do so. The presentist simply does not agree with your assertion that "I cannot regret a non-existent action." The presentist would re-formulate this as, "I cannot regret an action that never existed." The presentist could also turn this argument around; substitute "miss" for "regret." I miss [x] that once existed but no longer existed. [x] could be a dead relative or friend, for example. But you can't miss something if it still exists. If presentism is false, my dead relative still exists. So why do I miss him/her?

I don't see how you can reconcile the conclusion of the argument with divine simplicity. So a proponent of divine simplicity must deny that (c) follows from (a) and (b). I am inclined to agree with you that it is an unanswerable mystery how this can be so. From what I can tell, the best argument on offer from proponents of divine simplicity is that the same intrinsic act within God can have different contingent effects. At least that is how I interpret Feser's argument here: https://edwardfeser.blogspot.com/2019/08/a-further-reply-to-mullins-on-divine.html.

I now see that I was using language too loosely. By "necessary," I meant "non-contingent." With that clarified, my question remains: If I agree that God is either necessary or impossible, how am I to decide which is the case? Suppose I am not convinced either way by non-empirical arguments as to God's possibility. Can't I take into account miracles or religious experiences in assessing whether God is actual?

Thanks for this post, Bill, responding to my inquiry. As I've thought about this more, I wonder if a qualification is in order: assuming Anselm's Insight is correct (and I think we agree it is), empirical evidence might be relevant insofar as it has a bearing on whether God's existence is possible. For example, if you find the philosophical arguments on the possibility of a necessary being to be inconclusive, it arguably makes sense to consider whether empirical evidence can "tip the scale" in favor of one conclusion or another. But what doesn't make sense is to (1) accept Anselm's Insight and (2) grant the possibility of God's existence while denying that God does in fact existence based on one's weighing of the empirical evidence (e.g., Russell's famous comment that, if questioned by God about his non-belief in the afterlife, he would claim as an excuse "not enough" evidence).