First, sorry for your circumstances; there's nothing fair about any dementia, but the younger it starts, the less fair it seems.
Memory clinic teams are, sadly, all too familiar with these tricky situations, so I wouldn't worry too much about it. When I went with my mother to her first appointment I thought it would be tense (Mum was, shall we say, uncooperative and only going to prove me wrong!) so I wrote to the specialist beforehand, giving examples of why I'd "encouraged" Mum to see him. He made it clear at the appointment that he'd read my letter, but didn't refer to it in front of Mum (it was her appointment, after all, not mine). However she did so poorly at the questions he asked her that there was never going to be any doubts, so I don't think my letter made much difference.
In your position I'd put a short list of your main concerns down on paper that the specialist can quickly scan through. Handing it over in front of your mother might be a bit awkward, but I suppose you could quietly hand it to reception when you get there, and ask them if they could make sure it gets to the appropriate person before you go in. That way you don't have to worry about trying to have a private word.
Best of luck to you all getting through today. It's a tough day to face, but it's the start of a diagnosis, and that opens up doors, even though they're doors nobody ever wanted to have to open. My mother was heartbroken by her first assessment and the rather blunt prospects laid out for her. But there's no easy way to do these things, and I was relieved -- in the worst possible way! -- to know we were starting down the appropriate path to manage her situation.