Question regarding "ne"
If "ne" precedes a verb, is it necessary for the past particle to agree?
Yes, most often, and many grammars list it together with the third person direct pronouns among the mandatory cases; however, there are exceptions. The main one I can think of is when it's coupled with another object: "ne ho comprata abbastanza" (I bought enough [of it], of a feminine noun) but "ne ho comprato una scatola" (I bought a box [of it], might refer to either gender).
"ne" is a pronominal particle and implies the pronominal subject of the sentence. ex.: (io) ne voglio un pò