"As you shift octaves, all the notes cross hands and may be played with different fingers"

This means that you can add ornamentation from an octave above or below without needing to use the same finger in two places.

"As you shift keys, all the fingerings shift (relative to the tonic)"

As a former pianist, that's what I'd expect and do not find it a problem. Changing from G to D, for example, does not require a complete re-learning of the fingering, just memory of which note sharpens.

"On English concertina you have to finger each note in a chord separately for all chords"

This has the advantage that you can play a variety of inversions of a chord using the permutations - ideal for adding interest to a simple chord sequence.

I have a metal-ended treble concertina which is ideal for tunes, sessions and morris. I have recently been looking hard for a mellower box for song accompaniment. The English is really good for this.

My personal experience is that, while a baritone provides accompaniment a clear octave below the voice, the reeds in some lower-priced baritones are slow to speak - runs between chords can be a problem, as can staccato chording for up-tempo songs. As a result I'm looking more at a mellow-toned treble box and hoping to find a tenor-treble I can afford.