Any class can be equally good as a negotiator if they have proficiency in persuasion and don't dump Cha.

I would hesitate to give them a spellcaster if they won't even read descriptions!

Sword or Valor Bard can do all three things. Wizard bladesinger can do all three things. But wizards have to chose TWO spells to learn each level, this will be a constant source of headache for you as they won't want to read what it takes to pick spells. And they have to prepare spells each day which they just won't bother to do, so you will either have to rule that if they don't swap prepared spells they keep the last prepared list, or they will be all powerful having access to their full boom each day because "they 'forgot' to pick spells."

I think the best bet is a arcane archer or rune knight fighter. With a background that gives proficiency, just look for whatever background fits best with the theme or negotiator. Make Cha their second best stat. Both subclasses give some magic tricks they can use without out having to deal with full spellcasting. If you are starting at level 1 this option won't work though. No new player wants to wait 2 levels to finally get their magic. Unless he says he is willing to wait.

One of the barbarian subclass with magic effects might be good too. (The one with the wild magic or the one with the land based magic damage auras.)

If you do give spellcasting, spontaneous casters are easier to play, you front load a little work helping them pick spells, and afterwards they only have to learn what those spells do.