Holy trout that was a little hilarious. Damocles Photon seems to be doing half of your max HP in damage so doubling them before it hits was a surprisingly bad choice.

Songstress Songs do indeed stack up, you can stack them up to 10 times, the only exceptions being Perfect Pitch and Matador's Song which have their full effect after only one use. The songs are pretty powerful though, stacking up the defensive songs reduces the damage you take pretty massively.

Never knew Carnival Cancan heals 1/4 of your current HP either. Interesting stuff.