what are the differences between -んな or -んじゃね, and the similar sounding ん parts of わからん and わかんない?
firstly, わからん is a slang form of わからない the infinitive negative. is ん literally a lazy omission of ない and not an established conjugation?
secondly, わかんない just seems to be らない replaced with んない, again, as a lazy omission, not a conjugation consistent amongst other verbs. is this correct?
the ん in ふずけんな seems to be a recognized consistent conjugation of VERB STEM + ん however this ん is not a lazy omission for ない but instead の. any verb can take this form such as 買い被ん which i've seen used as 買い被んじゃねえ meaning don't overestimate me. する has a different conjugation of する + ん or するん like in するんじゃねよ meaning don't do X. the use of の or ん in these cases are form emphasis.
The negative ん verb ending is historically actually a variant of the negative ~ぬ verb form rather than the ~ない form, but since ぬ and ない are functionally the same, there's no harm in simplifying it down to "ん is short for ない".
る and ら can be simplified to ん when followed by another N sound, so something like わかんない is the negative form of わかる with ら→ん, and ふざけんな is the prohibitive form of ふざける with る→ん (not の).
The ん in するんじゃねーよ is a contraction of the explanatory の particle, as in んです, and has nothing to do with verb forms. A plain form verb followed by んじゃないよ is another way of telling someone not to do that verb, but it's not really its own verb form, just an expanded usage of the explanatory の.
Theoretically any ichidan or godan-る verb in prohibitive form could be slurred into ~んな instead of ~るな, yes. It's not its own conjugation, it's just a corruption of the pronunciation that you might hear in very rough or casual speech.
Pretty much every aspect of Japanese is variable depending on social context and hierarchy and levels of politeness. There are many ways to tell someone to do something or not to do something, that all have different places on the axes of soft/rough, polite/rude, formal/casual, etc. I would say the prohibitive is stronger/rougher/ruder, but both the prohibitive form and んじゃない (or じゃねー in slurred speech; note the vowel in ね is always extended) would be inappropriate in most contexts.
As I mentioned above, んじゃない isn't a verb form, it's the negative form of the explanatory の particle -- meaning that something like するんじゃないよ is technically a declarative statement of "You aren't going to do that" rather than an order saying "Don't do that", and the context of the situation is what makes it understood as a prohibition. The explanatory の has a far, far broader use than telling people to do or not do something (most sources would not even list that among の's usages), and would require specific context to be understood that way; the majority of the time するんじゃない would be understood as an explanation that you or someone else isn't going to do something, not as an order.
