I think Columbo is doing it for effect: when he's talking to Mario he already knows that Paul Gerard is the murderer. And since Gerard can't speak Italian, Columbo is sort of play-acting with Mario, asking him regular questions but seeming like he's playing "bad cop." All to make it seem like he DOESN'T really suspect Gerard, of course. That's always been my reading, anyway!
- The Wine: I think the point of putting the poison in the opener was to ensure that regardless of which wine Vittorio drank, he would be poisoned. Gerard suggested a cabernet, which Vittorio rejected in favor of a merlot, adding further credence to Gerard's alibi (not that there would be much way to collaborate it with Vittorio dead). I'm sure, having dined many times with Vittorio, Gerard knew he would have some kind of wine with the dinner.
- Mario character: The scene at the funeral drives me crazy, the way he stares askew while Columbo is watching the check being passed along.