Going back to the OP 2 items are mentioned.

Folk Club etiquette is set by the folk club organisers and has been discussed at great length on other threads. Each club has its own rules and as long as these are clear and obvious to newcomers no problem.

To avoid the arguments should we split sessions into 2 or more types, freestyle and controlled? Those with a lot of singing, those with a little singing, those with no singing?

At a freestyle which is what I mostly attend there is the occasional song and, unless the singer makes it very clear otherwise, it is accepted that joining in is the norm, with chorus, harmony, instruments.

While I'm on, what p****s me off most is those who try to change the tempo once somebody has started a tune, particularly those show-offs who like to play tunes at breakneck speed regardless of fitting the notes in. I have been known to tell them in mid tune, especially if there are some beginners in.