Ian, I'm not sure I understand the problem with these singers you refer to. Are they attempting to perform with their own accompanist, but are unable to alter their style, or are they in a singaround where other would-be accompanists are unable to join in? If it's the former, then they need to practice singing accompanied, but if it's the latter then IMHO it's up to them to decide how they want to sing it and the accompanists should know when to shut up and leave them to it.

Nothing is worse than having a song which you've worked on hi-jacked by someone who thinks they know better than you how you should perform it.