this seems to be a typical Iranian chaos. I have not heard it yet - and these (Ramadan) days it is hard to find out the real facts.
Since I (German) live in Iran for almost 7 years now I can only try to make you look at this sort of things in relaxed mood. In Iran nothing is possible - and everything at the same time. Iran is THE country of contradictions. This also applies to the rules and regulations. Of course you can run into problems that seem to be impossible to overcome. But, if you keep on trying, using different approaches, act (most important) friendly, respectful and on the base of common sense there might also be a solution for citizens of the US, Canada and UK to go without tour guide.
Maybe agencies make their profit out of this confusion. Therefore for those of you who travel overland, I would give it a try at the border when you get there. The border police is not over-informed, not perfectly educated, - and they are Iranians: be friendly, tell them you want so visit their amazing country, have come alllll the way to here.... - and I am almost sure, that they will make the difference because of hospitality, friendliness...