Why sais Frodo in this greeting omentielvo? Omentië means "encounter" and -lva is a suffix, possessive, first person, plural, inclusive. So why is it necessary to add an o, respectively to replace the a? Did I understand it right that omentië is now both genitive and possessive? If it is legitimate in Quenya to mix the cases, where else will it be necessary? Does a rule for that exist?
The "-o" is necessary because it is the genitive suffix. With it the possessive "our meeting" ("omentielva") becomes "of our meeting".
Ok ... so the Possessive Suffixes (-nya, -lva, -lma, -nqua, and so on) do not define the case, but -va and -ve are necessary for making possessive the word's case?
Both the pronominal possessive endings '-nya'/'-lva'/'-lma'/etc and the possessive suffixes '-va'/'-ve' can define the possessive case. The pronominal possessive suffixes are used when there's a pronoun modifying the noun in the possessive phrase ("our meeting" - "omentielva"; "my house" - "coanya"). The suffixes '-va/-vë' are used when there's a noun modifying another noun in the possessive phrase ("róma Oroméva" - "Oromë's horn"; "rocco i vendeva" - "the girl's horse").