I'm honoured to help. I think I can be properly impartial, because I don't agree with either of you; I don't believe in free will at all. We can do what we want, but we can't choose what to want.
Just a quick note before I begin - isn't "Christian theist" partly tautological? Is it possible to be a Christian and either a deist or an atheist? How would a person of either combination reconcile the supposed miracles of Christ?
Plus, a quick glossary for those not used to this kind of discussion: omni-max means omnipresent, omniscient, omnipotent and omni-benevolent all at once, as Christians claim God is. P1, P2, etc. are premises from which conclusion C is derived. If the combined P doesn't lead to the C, it's a bad argument.
You can do better than an unknown hypothetical OM/FW harmoniser. There are known hypotheticals out there. For free will to exist, let's go large and say it must be possible for any human to make any choice at any time, even if that choice may not agree with a god. One solution which also allows an omni-max god is that all choices agree with that god; the nature of benevolence is that all human choices, regardless of the suffering they may cause, are ultimately beneficial. The god has used its infinite mind and intuition to predict all possible futures, and has found (or declared) all of them to be good. This is a potential solution to the Problem of Evil as well.
Therefore I don't think there's a deductive path to a contradiction between an omni-max god and free will, so if you're going to argue it anyway evidential and probabilistic arguments are the way to go.
To add my own spin, the fact that there are answers to the tough questions like supposed free will and the Problem of Evil does not help Christianity as much as you'd think. The problem is that there are too many answers coming from the one religion. The PoE is the better-known example. Answers to that can involve exempt demons, free will, "mysterious ways" and the nature of evil itself. It would sound better if Christians settled on one answer, because the current multitude demonstrates that nobody actually knows.
Deny: "to state that (something declared or believed to be true) is not true". Dude didn't have a leg to stand on.
I don't know about the worst attempted proof I've ever heard (probably something about impossibly pretty sunsets), but here's my favourite bad one, courtesy of a well-known YouTube evangelist.
Here's a paraphrasing: If I say something will happen after an infinite amount of time, will it ever happen? No. Could there have been an infinite amount of time before the present? No, because if there were then we would never have reached the present. Therefore there has only been a finite amount of time, and God started it off.
The central problem is that infinity can end at a given instant. An infinite amount of time can have passed before the present if time extends backwards to negative infinity, rather than starting at a definite point. The same way positive infinity can start at a given number, e.g. (4,5,6,...), negative infinity can end at a given number, e.g. (...-1,0,1,2,3,4). Imagine two arrows starting in the same spot and pointing in opposite directions.
There are other issues of course. Even if the universe were finite, there are plenty of theories for its origin which do not involve gods (and often point to very eccentric properties of early time). Even if it proved a god, it provides no link to the Christian god advocated in the video.
However it's the way the argument makes a pig's breakfast of high school mathematics which really stands out to me as an occasional maths tutor. You have to wonder whether the guy genuinely doesn't get it, or he just hopes the video will reach others who don't, in which case the video is pure sophistry. Considering that he doesn't allow any critical comments, let alone official video responses, I tend to think the latter.