Regarding pH of coffee grounds, to my knowledge, coffee can be used
as a pH indicator and is only midly acidic. I do not know if the
tannins and theobromines, etc are a problem for the plants we are
discussing, or for that matter, the molds that can grow on this,
which often make toxins. The pH does not itself seem to be a
problem for most vegetables. With rotting, the pH could easily
end up very different. I have not gotten motivation to measure pH
on smelly rotted compost, certainly not with my expensive pH meter.
However, there are pH indicator dyes that can cheaply be added to
give a good approximation.

Dominic