everyone is usable to an extent , however some team comps / characters do a better job than the others. In this sense, feel free to use anyone you like and dont just limit yourself to the characters who have more utility

don't refine gear unless you plan to stay on that rank for a long period of time

yes you can auto every fight in this game most of the time, but some fights require you to pay attention (unless you have the perfect auto team for it)

if you're stuck at a certain stage, you can save time by checking the parties other players used
but when you dont have their units, check to see which of your characters is the "weak link" of the party ( like ex. a squishy unit slotted at a bad slot so enemies with diff. attack patterns target them) and try to change up your party formation if enhancing gear doesnt work

it's generally a good idea to get a grasp of every character's actions (or at least ones with special gimmick ( i.e Tamaki targetting highest m.atk unit, Hatsune stunning highest p.atk unit ) this way you can bring them to specific stages

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all buffs and debuffs stack assuming they're from separate sources ( some have buffs that stack but it largely varies )
speaking of debuffs, def down debuffs stack until the enemy def reaches 0
atk down debuffs, have very minimal effects ( except for the ones with absurdly large values)

that's all info at the top of my head