I'll try to answer as best I can
The Mohajer-2 is definitely the easiest to identify because it's the most numerous and the amount of data availble on it. Versus the other Mohajer series, it's defined by it's cigarette shaped fuselage, landing skids, foreward mounted ball camera, and straight wings.
There are however two areas which seem to vary
a) the nose - some have static, foreward facing cameras in them some don't.
b) The wingtips, on some it appears like the wings are simply straight (as in your #1 and #2 pictures), at a 90 degree angle to the fuselage and that's it. However, on others, it appears like the wings are slightly swept at the tip, or have some flourish (as in your #4 and #5 pictures).
These differences originally led me to conclude that the versions with the camera and the slightly-swept wings were the Mohajer-3 simply by process of elimination.
However, recently i've read that the Mohajer-3 has wheels instead of landing skids which would indicate that the above variances are just that, variances of the Mohajer-2 rather then being a completely new generation. This would of course indicate that we've never seen the Mohajer-3.
With regards to your #1 picture of the Mohajer-2/4 - i think it's just a Mohajer-4 that has been mislabeled as a Mohajer-2. I have no evidence for this, but i'm just defaulting to Occam's Razor on this one - i just don't think there are any other credible explanations. This isn't entirely without precedent either, for instance, i've got a picture of a BTR-60 floating around labeled as a BTR-82.
As to the naming issues in general, you've really hit the nail on the head of what is a really frustrating issue, that is, the tendency of the Iranian military establishment/industry to issue multiple names to single projects, and then reassign those names to similar projects in parallel development.
As far as i know, the Mohajer-2 has no alternate names.
The Mohajer-3 is also officially known as the "Dorna"
The Mohajer-4 is also officially known as the Hodhod. However, as your pictures indicate, there is also the "Shahin" which is identical in appearance except for various antenna and payloads, which is considered usual in display models and occurs in the Mohajer-2 as well. Of course, the Hodhod is also the name of this drone with the canted double tail boom.
The Iranian version of the Lark has a couple names the Shekarchi, Chamran 2, and possibly as the Toofan. Very confusing!