If a statement is merely about the future, I think it is simple enough to divide the timelessness of the observer, who could know such things (God), from the timeboundness of the actors. Future tense statements about what will happen will be grounded in fact when the time rolls around. Our access to truth in these cases is a matter of faith. God's access to truth in these cases is from his timeless observation platform, where, for him, the future is real and every bit as accessible as is the present or past.
The case of "what-ifs" statements is not so simple. They never come to pass so never have the opportunity to become fact in reality. Yet, God speaks as if they are not the mere musings of possibilities, but as if they were settled verities, as certain as anything can be. Could they be words spoken by the Lord that return void? No, it is impossible for God to be impotent, in error, or to be lying. If these kind of statements are true, they can only be so because God said it and God does not lie. Of course, that opens whole 'nother can of worms.
If God can make true counterfactual statements, it means that God's omniscience cannot be merely observational, as if God must await the decision of an agent before he can know what that agent will do. God's omniscience must be of the sort that is perfectly analytical and predictive. That raises the specter that whatever an agent will or could do is ascertainable by presently (or formerly) existing facts. That, really, is a deterministic framework: A leads to B leads to C, etc. Follow the trail backward and eventually you end up with a thought in the mind of God and a decision to make it so, or so it would seem.
Do true counterfactual statements demand determinism? No, because that leap is made without understanding how God actually knows what he knows, and ignores what God tells us about what he is and isn't responsible for. I do not think it is possible for us to formulate an understanding of how God knows what he knows--it's too far out of our plane of existence. We know what he tells us about his deliberations, nothing more. He tells us that humans do things on their own, things that never entered his mind to tell them to do. Sin itself is the consequence of an agent truly having the ability to make a choice counter to God, not initiated by God.
Therefore, the grounding for counterfactual statements is nothing less than God's character. I can make counterfactual claims, but they'd always be taken with a grain of salt. How many such claims made by blokes like me have proven out when conditions made them actually possible? It seems to me the problem arises, not in the grounding of such truth, but in how it could be so and still allow for human freedom. There is no puzzle here, truth is not about anything other than what proceeds out of the mouth of God! The question for us comes down to brute faith--we're forced to stand in the simplicity of a child and answer, "because God says so."