Odo has absolutely always returned the cart. In fact, I'd say he's tracked down everyone he can find that hasn't returned the cart and forced them to return one in front of him.
Nah, Odo believes in rules and order too much to force others to return their cart when there's no rule saying they need to do so.
He doesn't like it when people leave their carts in random locations, which is why he leaves his very neatly where it's not going to be a danger of rolling into a car or the middle of the lot, but he definitely isn't going to return it himself when it's someone else's job to do so.
Also he will arrest Jake and Nog and drag them into detention for Nog's quarter scheme. Quark will feign being outraged but then will insist he be included next time so he can teach Nog how to not get caught.
Nah, Bashir would devise a clever plan to everything work out just fine without people returning their carts.
Also, I'm pretty sure you got Garak wrong. He would promise never to return the cart, make a loud point about it for everybody, and insist on the subject every chance he gets. Just to covertly return it when nobody is looking because when Cardassia befriends the supermarket his people would benefit from the carts being correctly parked.
Bashir ended up helping several patrons with cart return based injuries and forgot to return his in the process.
He also invented a cart injury vaccine while helping, but it was shuffled into obscurity by section 31 who had several cart injury assassinations planned.
O'Brien returned his cart, and yours, and was mumbling complaints about lazy people while gathering two more when somebody drove a motorized cart into him and broke his ankle.
Kai Winn: grabs you by the ear and declares that the prophets dictate you not only return her cart for her, but that you must unload her cart into her vehicle. And if you don’t, your pah will suffer, my child
Changelings/Founders: "It's a flaw of the solids... To turn something as simple as food transporting - into a fundamental flaw of a decrepit society. Their so-called "cart" -- a mere extension of the hands, solids fail to recognize as part of one's primal nutrition gathering needs in order to just survive. But instead, as logical weapons to other solids who oppose their incomplete transportation rules..."
Morn is the guy whose job it is to collect the carts and stock shelves, but the only thing anyone ever sees him doing is smoking out behind the dumpster.