Lidy brings you the easy way to solve your session gripes:
Campfire training is the way forward!! When i was a Girl Guide, I was subjected to campfire singing (as a leader, i quite enjoy it now!!). It's simple. Someone important who thinks all the Guides like them (when in reality they are the most disliked authority figure on camp), stands at the front and leads the songs that only they want to sing. Occasionnally some young leader's boyfriend (when I was a young leader, it was normally mine, poor lad) who has been dragged along will try to join in on a guitar whose tuning has suffered awfully from the cold and damp. Guides will sit around cringeing for ages, wishing they could leave and go to the Scout camp in the next field, before deciding that joining in may be the only way to end this ordeal. Everyone suffers severe vocal damage from singing too loudly around an open fire on a cold night. Thus solving the problem of them being noisy in the minibus on the way home.
After that, everyone would find even the most hostile sessions welcoming and enjoyable! And they would realise the value of every individual instrument and the beauty of traditional songs. Simple!
Maybe the next camp I run could have an "In the Tradition" culture of the British Isles theme...