Thanks for your replies; its the simplest things .. and shows the importance of a schematic even for a simple circuit like this. Sorry - I should know better!
FYI the buttons are active LOW and the code I unashamedly cribbed from the mouse example has active high.
Fixed that and the code works; left click, click and drag and right click all OK and it no longer interferes with operation by the original mouse.
One problem resolved and a few ahead.
@JohnRob @Paul_B yes I know - but with a Micro I can adapt the program and button response to best suit her condition, such as change the debounce time to cope with tremors.
The next stage (OK its "simple") is to add a button to provide a double left click - which she cannot do unaided.
and @pylon - I'm not taking offence at your secription of my debounce as "buggy", I'd like to know what makes you think that. It DOES work.