Answering as a rather infrequent user of 'make patchcheck', but someone who vows to use it more often in future... :)

(1) Well, it would be awkward to use grep or wc on Windows, so it's convenient not to need external tools.

(2) +1 to a more detailed report, at least giving the file name and what was wrong with it ("trailing whitespace in yourfile.c");  I could imagine that people might care about finding trailing whitespace or tabs, but not about line length, for example.  (Many of the existing C files already have lines >= 80 characters.)

(3) Don't really care:  emacs already does both these things for me nicely.  I'd rather just have patchcheck do the reporting.  And automatic replacement of tabs with spaces in C files might not guess correctly what the author intended.