Sound engineering: Level down piano on live choir track with inverse phasing (line in track) and improve audio quality (Noise, compression)

I recorded my kids choir concert (30min) stereo with two condenser microphones. I accompanied the choir with a digital piano (using an amp), which was recorded by the stereo mics obviously. I recorded the piano with a line input too. Since the piano is sometimes too loud in the mic recordings, i would like you to kinda reduce the piano level by using inversion (i guess), since i have the clean "original" piano track.

Some decent noise reduction would be great too and compression too, since the digital piano (nord stage 2) has a very strong attack and some kids yelled instead of singing.