I've heard you out. Nice try, but you'll still have to answer to God for defending this ghastly individual. ;)

Your defence seems mainly to be based on what other teachers did questionably or wrong, rather than anything Umbridge did right. In my view, she remains a vile, sadistic, power-crazed bureaucrat fully deserving of audience loathing, partly because we've all encountered Umbridges in our own lives.

One point I will concede: Harry Potter rises to the challenge of Umbridge admirably, and her antagonism did shape him and strengthen him. But unlike the case of Snape, there was no learning-life-isn't-fair intention in her actions. Her motivations were entirely malicious.