Unclear. (1) "I'd like to overwrite a with whatever is the highest number in the parenthesis" – Does this mean that from all files with filenames of the form
a (N) you want to pick the max N and then mv 'a (Nmax)' a? (2) "And this needs to recurse" – Does this mean you want to seek for a (Nmax) in subdirectories, to repace a in the current directory? Or for each subdirectory separately you need to replace a with a (Nmax) found in the same directory? Or what? (3) "with the total output being a" – What does this mean?