It seems like Finn sword was disagreeing with Finn signing the duet with hole

No. The Finn Sword nods his head rhythmically while listening

I think that the Finn Sword disagreed with Finn singing that song because it expressed his sadness, and because of that sadness I think that Finn is going to wish to go back to moment that he met Martin at the citadel. In that case, the Finn Sword would have never existed, and that'd be the reason of it disagreeing. I guess it's what's going to happen in Preboot. Or maybe not.

Whats pre boot

An upcoming episode (S7 finale)