In most states, "pig buffer zone" laws have been stricken because they all violate 1A. However, some states haven't lost in court yet. In most states, "interference with the police" requires a physical act or explicit threat, not a particular distance.

You can tell the cops to go fuck themselves. That's covered by Cohen v. California, going back to 1971, and other related cases before and after that time. You cannot assume that "any volume is OK in all situations". But out on the sidewalk during the day, it is typically true that yelling "fuck the police" is protected by 1A.

Yes, the one finger salute is entirely protected. If you give a pig the bird and they arrest you, they'll try to find another charge to cover up the badness, and if they fail then you will probably win your civil rights lawsuit.

Remember, though, that the pigs don't care about the law. If they want to arrest you, they will, and as they say, "you can beat the rap but you can't beat the ride".