“If you insist there be...” , is that correct? Can I use it like that?

An interesting question. The be form is certainly grammatical in an utterance like "He insisted that I be at the meeting".

In such a case the verb to insist means to demand or require that something happens in the future. The be here is an example of the mandative subjunctive.

However, to insist also means to state or maintain very strongly that something is the case. For example:

He insisted I was at the meeting, but I don't remember anything about it.

In such a case, the mandative subjunctive is clearly ungrammatical:

*He insisted I be at the meeting, but I don't remember anything about it.

The utterance "if you insist there be someone being judgemental" is using insist in this second sense of asserting forcefully. So it certainly could be written as:

If you insist that there is someone being judgemental....

But is it ungrammatical to use a mandative subjunctive in a clause that does not follow a mandative verb (or adjective, e.g. It is vital that he be at the meeting)?

I don't have a definitive answer to this question. The sentence seems slightly suspect to me as a British English native speaker. But a quick Google search returns a couple of examples containing the insist there be string in which insist seems to mean maintain or assert:

• If you insist there be no nations in their mental map, they're going to identify with smaller identity groups.

• Regardless the age of the child or "child" (if you insist there be a distinction), this man has allegedly helped take advantage and abuse minors .