So the nominative plural (ye) was replaced by the accusative plural (you) first?
Only later did the new nominative plural overcome the singular.
I don't know. Both 'ye' and 'thou' survive in some dialects, so I've no idea which got pushed out first in standard English.
What were the separate dative forms before they merged with the accusative forms? Or would that take you too far back (like Old English)?
The dative and locative cases died out in the transition to Early Middle English. The Old English forms are all listed on
Wikipedia, and the dative ones actually seem more similar to our modern accusative forms than the old English accusatives.