No, you're not supposed to be able to get in there yet. I've fixed the problem in v0.1.2, along with a number of other bug fixes and adjustments. It's out now for early access patrons, and will go public June 29th.
Not yet. I'm planning on implementing a way to redo one time scenes like that, but it wont be until a future update. I'm hoping to get it done by next month, but I'm prioritizing making the game less grindy by implementing some other systems. Eventually you'll be able to replay all of her scenes by talking to her, including ones that get missed, but that's not until much much later.
She's supposed to be. She's trying to scam you. She won a recent popularity poll, so I'll be adjusting her stats to make her more realistically beatable in next month's update, as well as giving her another scene, but she'll still be the hardest in the game.
Yes, you can seduce the "landlady". Watch what she says at the start of the game.