Dear Captain (Dick) ! I do agree using one finger for one button is a good pedagogic aim and a technical one too (since buttons prefer being hit in their travelling direction) but the natural
lazy cheat like myself nevertheless finds it enormously handy using one finger for those fifths which are offered at proper distance.And even more - for your CEG (and similar) for instance I often use one finger for all three and for four-note chords like Am7, Cmaj7, Dm7,Em7, Fmaj7, G7 I usually use two fingers and sometimes only one for all four buttons.
This is facilitated since I have changed the buttons from 5mm to 6mm diam ones firstly for better touch comfort but also for this particular reason - to make it easier pressing more than one button with one finger.It all depends so much on individual habits.

Apart from that I think the finger plate should be eliminated - to allow the fourth finger to be fooling around with the others…