The following diagram shows the answer to the mechanistic question. Note that all the information applies to a single reaction sequence that you have been verbally lead through.
The curly arrows are drawn specifically to match the text in the question.

What about the resonance structures of the carboxylate ion ?

We can rank them as major or minor etc. by looking at maxiumum number of bonds and complete octets (e.g. I and III). If we don't have the maximum number of bonds (i.e. as in II and IV) then we have charge separation (which requires energy making the structure less stable).  Is this is agreement with electronegativity (e.g. II) or against  (e.g. IV).