1. Isnt minhag more chamur that din in some regards (i forget how it is brought down).
2. The source for eating in succah s”a is the gemara.
3. The mechaber brings down the minhag of those who only eat in succah during s”a day, but doesnt like the minhag. The nosei keilim explain the reasons for the various minhagim.
4. Several years ago i was a guest in a “5 towns” shul that did hakafos on s”a at night (i presume during the day to, but wasnt there). The rav announced all were obligated to stay for hakafos unless they were going home to eat in their succah. He wasnt saying it as a definitive psak, but was telling the mispallelim that they cant have it both ways. Either they are chassidish and their minhag is hakafos yes, succah no, or hakafos no, succah yes.
5. How does one not have a minhag regarding succah on s”a? The mechaber rules one should eat in the succah. Im not chassidish, and dont know the source for the minhag, but if your not chassidish and definitively do not have a minhag not to eat in the succah, why arent you following the psak of shulchan aruch?