Yitayningwut,
I don’t disagree that the assumption is that the Torah wants to avoid issur, but here we’re not avoiding issur, we’re removing the cause of the issur (the neder).
This is not a metziusdik’e chiluk, as you say; it’s a lomdish’e chiluk.
Nechomah,
Only one is necessary. If the oldest doesn’t do it, another one can. May Klal Yisroel be spared from tragedy.