Couple of things:
First, I don't see it as two situations in which both sides think they're going to get a player, and it could be either one that actually does. I see it as a situation in which Steve has good sources and gives good information, while Yancy goes by what recruits say to him and by inference. Steve was right on Elliott and has been right on many, many other players. Yancy has been wrong too often to count. Until I see otherwise, I'm going to assume that Thompson is in fact a State lean and Yancy doesn't know what he's talking about. If Ole Miss lands Thompson, then it will be as you said it was, but I doubt that at this point.
Second, I don't thing Eulls or McPhee will have any effect on Thompson's plans, at least they shouldn't if he's smart. Eulls is one DE if he doesn't switch to DT and you have to play at least two at a time. McPhee will be a senior and will have virtually no effect on Thompson's career, even if he doesn't redshirt, which at 220 pounds may be likely. If McPhee will affect his decision, then that actually bodes well for us since Ole Miss now has a commitment from Wayne Dorsey, who will be there 2 years as opposed to McPhee's one, and will almost certainly get Josh Tillman, another JUCO DE, as well. Throw in Bo Tillman from prep school last year and Mike Thomas from prep school this year, and Ole Miss has more bodies there than we do. We'll get Thompson.