The roster analysis was going so perfectly until the third party part, and even then, that was strong until the discussion of Sonic. It's not a knock against third parties that Sonic was the last character added in spite of his popularity, but an understandable oversight. Sakurai had decided on the roster very early during development, and only ever caught whiff of Sonic's popularity in 2007; fans began supporting him far more wildly after they learned that, because of Snake, third party characters were viable. SSBF brings up this same point in his Megaman thread's OP. I'll speculate further that Sonic was added so last minute because of the fan backlash that would inevitably occur if he was left out, and that, if the fans were listened to regarding his addition, their reaction would be considered in relation to his removal as well. As far as Snake goes, however, that's anyone's guess.
I don't think I've said so in this thread before, but I really enjoy the division of characters into your four categories, since those seem fully accurate; according to Melee interview analysts, Sakurai himself even divided characters between 'popular' and 'unusual.' It's also great to see that character archetypes are noted as advantages for their inclusion, specifically in the cases of Little Mac and Takamaru being a 'boxer' and 'samurai,' respectively.