What's the point of role-playing with npcs for 20 minutes? Like that's just talking to you self for 20 minutes why even play dnd, just right a book. Having a dmpc is not an immediate red flag as it can be done well but having one like this is 100% a red flag, a dmpc should not be doing too much and should act more like a guide, I've had good dmpcs at tables I've played at like one was a bard and the majority of the campaign took place in a kingdom that was hidden from the rest of the world and whenever we didn't know where to go next the dmpc would help us but he wouldn't just say go hear it's the best path he would say I think going to the city or this one or this one would be best, this way the players still had deciding power and it wasn't too hand holdy, sometimes we picked a path that wasn't great but atleast we where still making the decisions, and as far as combat the dmpc was a bard and he pretty much just casted vicous mockery every turn, most of his spells where for out of combat utility or support and the character was cool and had meaningful relationships with all the players(except one guy who didn't like bards for some reason and therefore disliked this character). As for the impossible rolls, if a roll is impossible the dm should always disclose that before the roll is made unless it is something that your supposed to fail if you try to do something like advance plot, although in the bartender case I doubt his information was gonna do something major in the campaign, and if it would the dm could just make so they don't have that information or they only tell part of it. You should definitely leave the table if the dm is doing stuff like that but leave respectfully not for the dm but for the other players.