It is not just a question of reaching individual buttons (my anglo has 40 keys arranged in four rows). It is also a question of playing phrases and chords. For me to play a phrase using all four fingers along most of the length of the rows and moving between the innermost and outermost rows usually requires me to move my hand within the strap. To change between chords may require quite a significant amount of movement.

I take your point about thumbstraps providing a pivot point, however I can quite comfortably achieve the same effect with simple thumb pressure. A higher rest may be an advantage, but much will depend on each individual's hands.

I've tried Geoff Crabb's 2-way air lever and it works well. However I've never actually noticed a problem using a conventional air button. The usual design seems to have been generally accepted for 150 years or so.

I'm aware that some people have ergonomic problems with concertina handles, and for them your proposal may be the answer. I'm also aware that Henrik Müller has adopted something similar for his home-built English concertina. However I find it is a solution to a problem which doesn't exist for me.