Most of these seem to be what etiquette writer Miss Manners calls “blather”: that is, pointless small talk not intended to be taken seriously.
Miss Manners permits responding to blather with semi-relevant blather of your own, or just a halfhearted chuckle or smile. My responses would probably be along the lines of:
Example #1: Cashier: “Are you having a party?” Me: “Ooh, would you do potato chips for parties? So salty.” (Yes, of course I serve potato chips at parties, who doesn’t, but this is mere blather: the pointlessness is the point.)
Example #3: Skinny woman […] (shouting after you): “Enjoy your ice cream!” Me: “Thanks, you too!” (Does she even have any ice cream? Not the point, this is blather.)
Example #4: You’re at a buffet. Person next to you in the buffet line: “We’re being so bad, aren’t we?” Me: “Oh my! Did you get some of that delicious-looking bean dip?”
What would be rude and nosy is if the speaker were actually trying to pin you down on the factual specifics of whether you’re having a party, or whether you’ll enjoy your ice cream, or whether you’re violating your dietary rules at the buffet.
Which is the point of the semi-relevant blather response: You’re treating the remark as just a content-free friendly gesture, not a serious inquiry that’s entitled to a factual answer.
And that’s why I really don’t know what to make of #2, where the cashier is apparently genuinely curious about whether you’re going to cook the gum and jelly together with the fish. That seems so weird I would probably just stay in blather mode, with something like “Oh well, doesn’t everyone?”
But a lot of people find it irritating to be blathered at about their purchases even if the blatherer isn’t seriously, and rudely, expecting to get a factual answer to their question. So I generally try to MMOB in the checkout line.