I think I can answer your questions about Simpsons vs MLP:FiM

The Simpsons, was never ment to be a 'pure' kids cartoon, it along with other shows FOX premired when it first started were more or less edgy/satire/dark comedy they were crude rude and in your face hit topics that most 'main stream' networks wouldn't touch with a ten foot pole with fear of offending the PC police. The Simpsons never packaged itself as a show for kids and to teach them 'life lessons' ect. In fact the Simpsons started as 'ad bumpers' on the Tracy Ulman Show before they spun off into their own series...a show with a decidely Adult Target Audience.

Compare with MLP:FiM a Cartoon Series airing on a Safe Kid friendly Network, packaged as a good wholesome show about cute ponies haveing adventures and learning lessons. Having a possibly (never stated or confirmed in the show) disabled character, a less perfect and pretty pony messing things up and being scolded by the 'pretty/perfect' ponies and no lesson about how this is not a good thing, can send very uncomfortable message out.

Combine that with a known slur against the disabled, and establishing that as the characters name...and combine that with the fact said name was FAN BASED...started by the Adult segment of the fandom IIRC...and well you can get how people would react badly.

So in short the Simpsons don't get the flack because it's considered Adult Satire rather than Kids Entertainment.