3) Actually, I think I addressed it, but I’ll explain. If learning shelo lishma is a mitzvah, albeit on a lower level, how can it not elevate him to a status of noach shenivra, and, as secretagentyid said, why would it be a sam hamaves? So Tosafos needs to split it into two categories. (The case of Acher might be shver, but a couple of suggestions come to mind.)
4) “Shelo lishma” is a negative – NOT for it’s proper purpose – so the meaning is actually the same. What fills that void is different, which isn’t shver.