Yes it just refresh the buff. If all songs are active, it casts the song that will expire soonest to refresh it.
You should use the duration that matches your dots. What level is your bard? Tune stuck in your head(I think) and the type 3 augs increase dot duration. For buffs I use conservative duration (less than actually duration), for dots I do the opposite. That's just a personal optimization preference.
Bard is lvl 125 with maxed out AAs. I thought the number was a "cast again in THIS_NUMBER seconds". But that is not how it works, I guess. Perhaps this is a no-issue when the queue is actually made up of more than one song.

And the dots have a duration of 0:12 (0:18), so I presume this means that due to level and items and AAs the dots land up to 3 times? I have never really looked to tell the truth. This just what I assume happens.
So, I did this. Thoughts? Opinions? Is something just flat out wrong? (There could be typos or other misstakes)
songIF=!${Me.Invis}
song1=Aria of Tenisbre Rk. II^30^1
song2=Blade of Vesagran^180^${Me.Combat}
song3=War March of Nokk^30^1
song4=Kindleheart's Chant of Flame Rk. II^18^${Me.Combat} && ${Target.Type.Equal[NPC]} && (${Target.PctHPs} > 40 || ${Target.Named})
song5=Swarns's Chant of Frost Rk. II^18^${Me.Combat} && ${Target.Type.Equal[NPC]} && (${Target.PctHPs} > 40 || ${Target.Named})
song6=Goremand's Chant of Disease Rk. II^18^${Me.Combat} && ${Target.Type.Equal[NPC]} && (${Target.PctHPs} > 40 || ${Target.Named})
song7=Marsin's Chant of Poison Rk. II^18^${Me.Combat} && ${Target.Type.Equal[NPC]} && (${Target.PctHPs} > 40 || ${Target.Named}
song8=Eoreg's Insult Rk. II^6^${Me.Combat} && ${Target.Type.Equal[NPC]} && (${Target.PctHPs} > 40 || ${Target.Named}
song9=Grayleaf's Reckless Renewal Rk. II^30^${Me.Combat}
song10=Pulse of August Rk. II^30^1
song11=Regar's Lively Crescendo Rk. II^30^1
song12=Arcane Rhythm Rk. II^30^${Me.Combat}
Regarding this: /medley queue "Slumber of Silisia" -targetid|${Me.XTarget[2].ID}
If my bard is smacking away with kicks and blades on the first target, won't switching target to cast mez, break said mez with the melee, before switching back to the first target?