Letters are from here.
Divorcing would-be biz partner: I think we can agree that while certain interpersonal traits are desirable in a person-to-person relationship, the same pressures don't exist in a business relationship as a marriage. Though I suppose he could think he's bored and be tantalized away with the come-ons of a sexy younger business (hey, wait, isn't that what you two are proposing to do together?). And a little deception and underhandedness is a bonus in today's market. So what I'm saying is: I think you're overthinking this. But if you find yourself not liking the guy, that might become an issue for your business.
Embezzling Bride: Insert "cost of average wedding is so high" joke here. I think she's not expecting any coworkers to show up at this point, though, to answer your question, so you don't need to worry about etiquette and not showing up. I mean, that's hardly a huge transgression at this point. If she calls (what gall!) to see if you're still on, just say, "You're kidding, right?" or maybe, "No, we don't really feel good about going right now." No justifications, just the facts.
Bulimic Teeth: Get them fixed. People know you've got a messed-up mouth anyway, by your own description, and there is nothing sadder than a 24-y-o who won't smile. I hope you don't have to have dentures (its own nightmare -- implants may be an option, and even though they're probably expensive, you're 24 years old and it's a long-term solution), and if people ask what was up with your old teeth, you can either tell them if you want or tell them it's not their business. Also, any dentist will want to help you get the best smile you can. You've got some self-confidence to shore up, baby girl. Get to it.
Friend married an old guy: LMAO!! Apologize to the friend. But honestly, you marry someone that much older/younger than you you develop a thick skin. I have a relative (my relatively youthful age) who married a guy who remembers WWII, and you think she hasn't heard it all (if anyone is an ass in this Facebook dustup it is the brother who liked your comment -- and you can rest assured by his assaholism that your mistake is hardly the worst she faces)? You're probably okay if you just send a message saying, "OMG, I'm sorry. I hope you two are happy."
Sister schtupping her Prof: The way you're writing this note makes me think you're the wrong person to say something to her. Why are you so het up about her sex life? I know, I hate to see good people schtupping the undeserving too, but seriously. To be honest, all you can do at this point is say, "Muffy, I am upset that you're having sex with a married man who is also your professor for all the reasons you have probably already thought of. You're going to do what you're going to do with your life, so that's the last time I'll say anything." Then for the love of God don't get dragged into her justifying stuff. Just keep saying, "I can't change you, you can't change me, let's just move on." Or you could totally butt out. But I say sisters have the option and obligation, in a close relationship where they want each others' opinions, to say their piece.
Ditched at the altar: Send an email to your manager explaining the situation and asking that people please don't make a big deal out of it because you are NOT in the mood to discuss it. I am so sorry this happened to you. But really, this is a simple question.
Feelings for BIL: Yeah, you are either both flirting with disaster or you're making up some drama in your head. If you love your husband you will put yourself on lockdown around BIL. Don't be alone with him. Don't obsess over the fantasy in your head about him -- just let it go. Let him go. Hold tighter to the one you really love and who is there for you.
Schtupper's sister again: Okay, this is a different context you've provided. Let your sis know that the grapevine is alive, and you're worried about the consequences for all involved, and you're worried about her. Do your best to bite your tongue about her dumb actions and get your concern across -- for her.
Baby daddy?: Oh man, this is the kind of q that got the Men's Rights Assholes out on the Fray board. Female covert inspermination, leading to duped male, the worst male there is to be, and it's all her fault and she needs to pay and he should never ever look at the kid and he shouldn't put one penny towards its growing up and not the other one, either, so she SUFFERS. EVERY DAY I'M SUFFERING. Anyway. Moving on. I would say that you need to quit obsessing because your baby has a father. I'm not sure what being honest with him about your past infidelity and current suspicions would bring to the table besides actual suffering, and not just by you but your children and your husband. Focus on what will make your family happier and healthier, which is letting this drop.
Naked teacher: Okay, wow, this makes that time I saw a teacher in the grocery store mundane. Look, some people your age are assholes, and would like to see a naked, hated teacher just to tell everyone about her saggy old person boobs and cottage cheese ass. But you are a locker room denizen, so you're cool and know that what happens in the locker room stays there. However, there is a difference between saying, "Hey, that was awkward. You know the locker room code?" and "I will get you." So tell your parents about her threatening behavior. And be cool, don't tell people at school what she looks like naked.
Allergic to everything: That sucks. Maybe there is some way to coordinate with hotels ahead of time so they use 7th generation or whatever? At least you can call them and have them note that you'll bring your own sheets if not.
Dog murderers: I feel enough doubt that people are willing to off a dog because it barks incessantly to say act like you know nothing. Really, it's the owner whose offing would accomplish what you need accomplished ...
Or you could just be sensible and do what Prudie says.