Stock behaviors and comments to adopt in situations like these:

Stand with a "marginally amused" look on your face while she talks.
Dig around your purse for that nail file you've suddenly needed.
Make eye contact and speak with as many people around you as you possibly can while she's ranting.

"Everyone is entitled to their own opinion"
"How predictable"
"My, what big teeth you have"

Just kidding on the last one (or maybe not).

I watch our BM gush all over DH and find it yakky inside my head but I won't let her know that. I know she's doing it only for my benefit. I keep reminding myself that this is only X minutes out of my day, my week, my life since most of the time I avoid the general vicinity in which she presides.

I really don't have much in the line of advice for you other than what B22S22 said, but I have to say, when I read your first couple of paragraphs, right up to and including her charging for 1/2 of her postage... I was picturing the typical trailer trash gal that so many of us have to deal with, who has no job, no education and is just looking whatever free goodies she can get.

Then you blew my mind when you said she is a lawyer!!!

Then you mentioned that she is 41 and has a 25 year old boyfriend... Sounds like my SS13 BM, she is 38 and has a 20 year old BF (same age as her 20 year old.. who is not DHs) I never realized that women had mid-life crises too! LOL

Am wondering why you allow BM to get away with harassing you via text/ on your phone? I am in the UK, but after the millionth time of my BM texting my DH calling a a c*nt, sl*t, b*tch, wh*re... and threatening me (she would do this at the drop of a hat e.g. one weekend DH had to work and ofcourse that was my fault) I reported her to the Police for harassment and told DH if she continued I would prosecute her. He passed the message along.

Don't know the US federal laws but am assuming there is legislation in place for harassment, for name calling which can be classed as threatening and as BM is an attorney it would go down like a lead balloon with your law society if she got slapped with a criminal charge. Our solicitors can be disbarred here - am assuming the same in the US?

You have her over a barrel. If FDH protests about this, then eyeball him and make him choose. Besides which this affects you, its abuse towards you so he can take a flying leap imo. I would also be cold hearted enough to get really upset about these events and ensure sd hears this, voicing the fact you dont WANT to report her, but you cant see what else you could have done (key word is DONE - dont give BM heads up over doing this, revenge is best served cold and with no forewarning). Even if you dont feel the victim, PLAY the victim. I find anger can get people's backs up but if you get gently upset people are more inclined to be protective (if this means your relationship with sd/FDH goes unaffected by you doing this then DO it). }:)

As for her going mental about you making changes in the house - what has been FDH reaction? I do so hope he a)either completely ignores her while bend over in hysterics toasting her lunacy with you (after all she can fit as much as she likes, doesnt change a damn thing and boy does that grate hence her anger and jealousy over what you did) or b) he has a blanket smug email he sends her "Dear BM, SM is my future wife and this is her home. Not yours, so its none of your business what we do or when we do it in our home. Regards DH"

Then ignore her other emails/calls. I would suggest your DH actually gets a phone that only BM has the number to, so that you can switch it off and record her VM's and threats to block access based on the fact your guns are locked up tight in a secure safe (which unless your 9 yr old sd is a cat thief and master locksmith) is unable to access. Am sure this would go down well with the judge when you take her arse back to court. All these VM's/texts/emails will make her look like a lune along with her criminal conviction of harassment (think restraining order as next step if the charge doesnt stop her).

My advice is to take this firm approach but they dont stop. Not when they are on this level of madness. Mine didnt, she just escalated it. So take charge if FDH wont and protect yourself and your family by doing the above, and then watch BM fall.

P.S as for this weekend, make sure you look understated hot (i.e dont look like you are trying hard) but outclass the bitch. Then ignore her. Not sure why if at all you have to pretend to be nice to this vile specimen and I would expect FDH to back me up in your shoes - his job is to protect you from this bully, not kow tow to her fakeness and attempts to strike at you at any given opportunity. Remind FDH you are in the position because he made a poor decision to procreate with BM and will be seeing BM on the weekend because you want to support sd, ok damage done and we all make mistakes so you arent blaming him but you are expecting him to make his allegances known even if that means snubbing BM in public and explaining to sd9 the reason for this "sd, I would like to be friends with your mother but her behaviour towards me and sm is unacceptable when all SM is, is nice to you and does nice things (insert lovely example). This is between me and your mom, not for you to worry about and if your mom says anything its ok to say you arent taking sides..."