Question: From what my daughter's boyfriend has told me of his childhood, his mom sounds cold, unloving, even borderline abusive. He's quick to reassure me that she has changed since then. But I can't unhear what I've heard.
I know the default is to be cordial when I meet her and give her the benefit of the doubt, but how do I handle it if she puts him down in my presence?
Answer: This is actually two questions. The first is how to handle what you've heard, and the second is how to handle what you witness.
For the former I recommend, yes, the benefit of the doubt, but you can't half-heart it, or else you'll take the slightest of her transgressions as license to believe the worst.
So try looking at yourself through this lens for a moment. Page through your memories of raising your daughter, and fix on a couple of your lowest moments. Times you yelled, times you acted selfishly, times you said something mean. Now imagine your daughter spinning these tales for a therapist. Yikes.
You may know these were deeply regretted exceptions, typical and human and duly mended, but you also need to know that, if phrased just-so to someone who wasn't there and doesn't know you, these could paint a scary picture of you. Of anyone.
When you meet the boyfriend's mom with that in mind, maybe you can upgrade your we'll-just-see to a truly open mind. Think of it as innocence until she proves herself guilty.
As for any mistreatment you witness, handle it as you would any other: Stick up for the target. Anything from a raised eyebrow to a full-out "I believe you owe X an apology" can let people know unkindness is unwelcome here.
Question: Often when my girlfriend calls me or I call her while she is with family or friends, she will announce that she has put me on speakerphone, at which point I am expected to converse with whoever happens to be in the room with her. I find it annoying, and I've expressed this to my girlfriend. She in turn finds it annoying that I am not more enthusiastic about speaking with her friends. Which of us is on the right side of etiquette, in your opinion?
Answer: Putting someone in an awkward spot is the exact opposite of what etiquette exists to accomplish.
But this isn't about etiquette.
You're trying to have a say in what you do, which is your right, even if your methods might be problematic. (You are asking her to change her behavior - which is OK occasionally, but not repeatedly, and not as your only solution.)
She, meanwhile, is trying to have a say in what you think and feel, which is a boundary violation. It's also bad for a relationship. For that matter, so is the aural cheese-grater that is a speakerphone.
Usually the best way to handle behavior as boorish as hers, since the chances are slim that she confines it to phone-bombing, is not to date it anymore. If you're not there yet, then change your own behavior: "Hi everyone! Girlfriend, call me back when you're free," click. That puts you fully in charge of what's fully yours to control.