>Can a song have two 'worst' repertoires?

Most don't, but yes, a song can.

>Does a song have 1 best, 1 worst and 2 'mediocre' or is there
>a 'good' and 'bad' repertoire in between 'best' and 'worst'?

Depends on the song, but most are one best, one worst, and two mediocre.

>Can your instrument quality eclipse the affects of singing a
>song in it's 'best' repertoire if the instrument is of high
>enough quality and you're singing in a mediocre repertoire?

Depends on the song... maybe. Generally I would say no.

>Do you choose repertoire based on instrument or your
>instrument based on a given repertoire?

I usually pick an instrument based on my repertoire when I'm playing a bard.