No, and yes. Homeless people aren’t second-class citizens, they’re normal people who can’t pay rent. All US cities have ~1% homelessness, including SF. So SF isn’t notably worse than elsewhere. But the primary cause is always lack of affordable housing, and SF has notoriously bad housing policy. Worse, SF has less shelter. NY shelters 95% overnight vs SF shelters only 16%.
No, diet soda itself will not make you fat. Diet soda does not have sugar. The reason you get fat from regular soda (vs diet) is because regular soda contains sugar. Both diet soda and regular soda cause your body to release insulin, which tells your cells to store sugar as fat. But since diet soda has no sugar, there’s nothing for your body to store as fat.