Just taking your first paragraph, the whole point of working groups is that they undertake the research into a project before it is presented to council for a decision. Their research should cover any questions anyone might have and like any project, none of us are likely to be experts in any subject but at least it should be possible to get answers and if there are still concerns, no decision should be made.
I personally can't see a problem with a presentation by a neighbouring parish on their experiences. In fact, I'd welcome it. I believe it's important to be as fully informed as possible before making a decision so am a bit confused at the suggested that just because it's been organised by a councillor rather than the clerk, it should be cancelled. I would argue that it is not a meeting but simply an opportunity to gain some information; something the working party could have organised for themselves as part of their fact finding but they've chosen to open it up to others. That is to be welcomed. No decisions can be taken at any informal presentation but it does give background information on their experiences so why is it not acceptable simply because someone other than the clerk arranged it?
Aside from the risk that the meeting might be particularly long, one solution would be to incorporate the presentation into your formal meeting as Mentorman has suggested. It is too late to change your agenda but as this is part of the agenda item to consider the proposal, I don't see the problem provided your standing orders permit the chair to invite public speaking as part of your meeting.