Earlier quoted context omitted.
> backspace on the start of the line doesn't seem to move the cursor at all. AFAIK that's controlled by "set backspace=" and thus will vary based on one's .vimrc (or the system vimrc, installed by the package manager): https://stackoverflow.com/questions/18777705/vim-whats-the-d... One can see the current value of the backspace setting via :echo &backspace
Actually, the 'backspace' setting only governs what the backspace key does in Insert mode. In Normal mode, which is what I was referring to in GP, has the behavior I describe regardless of how 'backspace' is configured.
I was inspired by this newfound "using unexpected keys for navigation" discovery and found that (at least in my vim 8.2) holding down space in Normal mode similarly travels across the EOL boundary, just as backspace reverses that trend, so at least it's consistent