Let me cast calm emotions first as you seem to have a pickle in the haystack, here.
There is a fundamental difference in our perspectives.
You're talking about saying "No". I, as a DM, will only say No in one circumstance - if the player does not fit in the game, the group will tell them, "No, you can't play with us." And this is something I've only done three times since the 1970s.
Whena player comes to me and says, "I want my hafling to get +8 to Strength and +7 to Charisma because it'd be funny", I will not say yes. I'll say, Let's talk. I'll explain why I think that would be bad for the game and ask why they thought their idea was good. I expect that we'll reach an understanding and come up with a gameplan that works for everyone. In the rare instance where the player is unyielding and disruptive, we talk as a group and decide what makes sense together. That could result in the group offering an ultimatum to the player - back down or leave - but that is ridiculousy unlikely.
Yes, there are judgement calls I, as a DM, am making on when to go to the rest of he group or when to accept a player modification idea - but you're acting like a fierce battle is a likely occurence when, in my experience, after playing with hundreds of different players over the decades, find it to be exceedingly rare.
Relax. Ask the players to work with you on a good story to address the mechanical elements they want *if* they come to you with a mechanics first perspective. As long as you get to good mechanics and a good story, you're in great shape. If you can't, suggest going back to the drawing board. If that doesn't work, maybe you're in that rare corner case where it isn't a good player fit. That will result in a great game almost all of the time.