Can't you fake http://conntest.nintendowifi.net ? After all, it's just a plain HTML page served over HTTP.
Probably, but most people probably won't go to the trouble.
Same combo on three consoles, not a single one banned.
Have you played USUM on the 1.2 update recently? Because the main thread I am seeing over at reddit is that most people who were banned were playing USUM, perhaps they added some cfw checks to the 1.2 update?