I'm a little late to this thread, but as to your initial query about "Piano to concertina," that describes me. My answer, ultimately, was a duet system (I chose a Hayden duet). Here you'll find the low notes on the left side and the high notes on the right, as with a piano, and a unisonoric system with consistent fingering patterns for whatever key you're playing in.
I started on an Anglo, but became frustrated with some of the limitations of harmony, and the strange fingerings required as I moved southward over the circle of fifths. The Hayden duet is quite consistent in how it plays from key to key. I've managed with my Hayden to play some of the simpler tunes from Bartok's Mikrokosmos, and even some simple Satie reductions. Here's a classical(ish) piece I composed for my Concertina Connection Hayden duet (42-key Peacock model):
Best of luck with whichever system you choose! They are all wonderful, in the end.