“...though their calling it “fiction” certainly nullifies its position as scripture.”
Song of Solomon is a lyric poem. It has also been described as allegory. Does that disqualify it as scripture, too?
The Song of Solomon is inspired scripture, and therefore it can be used for doctrine, reproof, correction, instruction for righteousness, that the man of God may be perfect and thoroughly furnished unto good works. When I said “fiction,” keep in mind I also affirmed with the modern RCC that it is “replete with historical and geographical errors.” We cannot consider something like that to be inspired by the Holy Spirit. If the Holy Spirit wanted to write a poem, He would not masquerade it as history.