Thoughtful questions you raise, Nick and sound advice from the catters too.
Another thought I have is that variety in a set always helps.

Although I'm a Child Ballad freak, I once heard six in a row from an accomplished singer who turned off the paying folks during the third song.
Sheena Wellington, however, will follow something like "Sheath and knife" with a wee daft song like "Beano and Dandy and Sunday Post".
The contrast between the performances required just helps; and fewer people retreat to the toilets.