Sure, he could. He could do like he did with Thomas and physically reveal himself to every doubter, or blind every anti-Christian on a road, or have every rebellious believer swallowed by a fish, or do whatever outward sign that would remove all doubt from every unbeliever. But clearly God is pleased by faith and has chosen that to be the means through which salvation is applied. "Blessed are they who don't see and still believe."
No, He couldn't do all those things, as it is obviously not in His plan to do so. So to clarify this OP, it would be better stated that He WOULD not do them, i.e. it is not in His Will to do so. I think you've attempted to say this in a sense, although your OP is feeble in essence and quite ambiguous.
If it were His Will to save all, He would have done so, but obviously it is not the purposed outcome of His plan, and even if He did do all of the things you suggest to all unbelievers, it would not be enough to save them anyway.
Belief alone is not salvific, and He appearing to some would only further expose their wretched hatred of and for God in many cases. The Gospel accounts for example?
Preaching the Gospel exposes both those who are His and who are not His sheep, just as it did in His day. By the way, salvation is applied by grace, not faith. Faith being a state, not an action, and is in itself the gift of God. Justification is applied because of this state, salvation by grace.
Even those of us who affirm "Contra-Causal" Freewill acknowledge that God has in the past and may in the future intervene with circumstances (signs and wonders or whatever) to ensure a specific outcome...(i.e. inspiration of scripture, crucifixion, etc)...but to suggest that EVERY circumstance, outcome, thought, evil intent, heinous crime and sinful act is likewise determined by God only serves to:
(1) undermine the unique and divine attributes of those acts which SHOULD be attributed to God and
(2) impugns God's Holy and sinless nature by making him appear culpable for originating sin.
Believing God has determined all things does not lead to your illogical points in 1 and 2. That is an unfortunate and pejorative misrepresentation of DoG brothers.
That one
doesn't believe He has predetermined all things in whichever will He has done so undermines His Sovereignty. This is what you have done and it is a fails to separate His Holiness from what He has determined. I believe you are looking at God in how we would look at a man (not that you believe God is man, just pointing out an error in your lens). For instance:
God = Holy + controls/determines all things = God is Holy still because He is Immutable, Perfect, Holy, Just and Incorruptible. None of this effects His perfect nature, it cannot, He is incorruptible, thus it is impossible, and thus He and He alone is capable of accomplishing all His purpose and will by His determinitive counsel and yet remain Holy, Just and Perfect.
Hitler = Evil + controls/determines all things in his power = He is evil, sinful in nature, and is a corrupt being, thus he is capable of becoming even more corrupt and evil by his very sin nature.
That He has determined all things does nothing to His character, He is Immutable, Perfect, Holy.
Your objective in the OP is to take a persons view of Gods predetermined will and Sovereignty, and caste it as demeaning to the character of God. I believe the secondary objective is to be somewhat demeaning toward those who hold this position.
I believe actually the opposite is happening as you appear to hold a very deficient view of His determinitive will and Sovereignty, and of His incorruptible nature in all of these things.