Learn English – can’t ever vs can never

1. No, you can't say "are not we all?". It is ungrammatical, as you suggest.
2. Yes, you can say either "you need not go there" or "you needn't go there".

The reasons have to do with negative contractions and the fact that they count as a single auxiliary verb. Only one auxiliary verb may be inverted by Subject-Auxiliary Inversion, which is required by Question-Formation. This may be a contracted verb like isn't or don't, inverted with the subject as a unit. If the not is not contracted, on the other hand, then it stays where it is, after the Subject.

Thus these sentences (the "Q" means "Apply Question-Formation")

• We are not all here. ==Q==> Are we not all here?
• We aren't all here. ==Q==> Aren't we all here?

follow the rule for subject-auxiliary inversion, but these don't, and are therefore ungrammatical:

• *Are not we all here?
• *Need not you do that?

In the case of (2), there's no inversion, because it's not a question, so the contraction can be unpacked at will.

Summary: Contractions are only optional in their original position. If they're moved, they're frozen.

In the example you have given, "ever" serves two purposes. It does act to emphasise the negative as you have suggested. It also adds a temporal dimension to the sentence.

If you consider your sentence without "ever" it gives the impression of a one time event:

I created a session but nobody joined.

Whereas your sentence indicates that this has been happening for a substantial length of time and implies that it is unlikely to change:

I created a session but nobody ever joined.