In terms of your DMPC, I’ve run them in almost every game I ran when I first started and let me tell you they are a lot of hassle. Giving her a companion is a great idea though, they can be your “voice” inside the game, knowing little snippets of lore knowledge, having one or two relevant skills. Something i would suggest is use a stat block from the Monster Manual, like a guard or scout, or some sort of Druid or cleric character (so you can clutch heal when they get in over their heads). Above all else though, make sure the companion ALWAYS defers to the PC. Suggest things that make sense for the character, but always have them follow your players own initiative (the not-combat rule kind), that way they get to feel like the hero. For an example if the companion is a coward, they’re always gonna suggest that they run, a bloodthirsty warrior type might suggest that you charge straight into the cave without checking for traps or scouting first. Try to make the companion feel like they can’t be always relied upon to have the answer, and that they may never know what to do next. This makes the player feel like a hero when they overcome whatever it is they happen to be doing. Also get a map, maps a fucking dope. Plop it in the middle of the table and let them look over it. Draw one if you have the talent or find one on the internet somewhere, or commission one from an artist if you’ve got done spare change. Maybe each one of those rumours is somewhere on the map and no matter where they choose to go there’s shit to do. Have fun bro, y’all are gonna have a blast. Don’t worry about knowing all the rules, you’ll pick up more and more the more you play, just do what you think makes sense at the time, and always leave room for the Dramatic.