Ive tried to read as much as I can, apologies if I repeat or missed an update.
I’m a mum probably of your mums age and a grandma. There’s two sides of the coin really. Your mum is clearly in the first flushes of love. Almost like teenage love (remember it?!). She’s digging her heels in because she’s so in love with him, this is “the one” in her mind. She doesn’t want to miss out on a special day without him. Equally your dad will be there with his wife, she doesn’t want to be on her own. It’s your day, you’ll be busy and apart from the duties she has with you, she doesn’t want to stand alone at the bar etc. She wants to enjoy the day with her partner, show him off, for him to a part of it too. Maybe your mum is an insecure, not so confident woman. Maybe she needs someone as support to her on a big day
equally on your side of the coin, it’s your day. It’s about you. Everyone should be supporting you on your day, let alone your own mum. You think your mum should be your biggest supporter. You don’t know him, you don’t want him in the photos, on the top table etc. why would you, you don’t know him and he could be gone in the summer! You feel your mum has enough of her side of the family to stand with, talk to etc
there is also another strand to this, you’re upset at how your mum has spoken to you. You feel it’s not on. Your mum on the other hand is frustrated, fed up and not had the response she thought you would.
so where do you go now? Firstly you need to talk to your mum about finding a way forward. You need to tell her that you don’t want to be spoken to like that. Whatever the situation now or in the future, firstly that’s the bottom line. She may not like it but really once someone tells you I want to move forward and let’s set some boundaries, really what is there to answer. Tell her you love her and need her, let alone on your biggest day.
as to the boyfriend, I don’t know. If the venue says he can’t come in the evening, he can’t come. What does your mum want you to do about that? You can’t change it. Can you allow him to come but he sits at the back of the church (or wherever), he doesn’t sit at top table, he’s not in the photos etc. really he’s in the background as much as possible.
personally as mum and grandma, I wouldn’t be pushing for him to come. It’s your day and I could see him at any other time or day. It’s one day that I won’t see him and my time and attention has to be on my family. It’s a big day for you and your mum, I wouldn’t be worrying about the boyfriend if I was in your mums shoes
hope you manage to sort it out