Aquiestoy - xwh here is likely undiagnosed Borderline PD with high NPD tendencies as well. He has used many lies and rewriting of history and twisting of truths directly to and in front of our kids, so I think I have some understanding of that with which you are dealing. (He's even tried to change history with our two youngest by saying he was present at an event when he was no where around - in fact, he was most likely with mow at the time instead! Both kids knew without a doubt he wasn't there, so it upset them greatly he would insist he was.... Add to that he seems to have also insinuated mow was there as well, and she most definitely was not, and well..... You begin to see how he is trying to replace me even in the kids' memories with mow.... Ugh... but I digress....)
Even with that, and despite that mow, with xwh's support, was filing false police reports and frivolous/false court cases against me, all with the goal of having me arrested/jailed, I tried, and still try, hard, to keep the kids out of it. (And mine were all older kids/teens through the D process, too....) It wasn't easy, and downright impossible at the end, as the only way to prove my innocence in one of the court cases, due to a fabricated document created by mow as "evidence", was to bring my oldest into the fray. (Really, mow/xwh brought the child into it, though I had to be the one to ask that child come to court, for my defense....) That said, I never discussed anything with the kids that I didn't expect may well get back to xwh; I also didn't tell them not to tell him something, save things that I said were mine to tell him at the proper time. (I didn't want him messing with my vacation plans, for example, so would discuss the planning with the kids, for their input, but would let them know I would tell their father when it is time that I should, per the order/agreement between us, but, for now, it is just for us to know as we plan.) I resigned myself that anything done around or with the kids would possibly get to xwh, in part as I felt it likely (and it was hinted at by the kids) xwh was grilling them just for such info. Anything I didn't/don't want getting back to xwh, the kids do not know, unless they absolutely must, and then I know I am just taking my chances on xwh knowing.
All that said, if my child(ren) asks a question, I answer it honestly, though limited to what was asked and not more; however, sometimes that answer has been "that's something that is for us parents to discuss, and not for you to worry about" or "I wish I could give you the answer you seek, and maybe someday I can, but, for now, I am, and your father is, constrained by the court order not to discuss it with you." If, though, I learn xwh has lied to the child(ren) about something, I do address it directly, without calling xwh a liar outright, and tell them the truth, or, often, ask them questions designed to get them to discover the truth on their own, if that is possible. (And, if it is something which is not supposed to be discussed with the child due to the order, I consider that no longer applying, since xwh had already done so - but I still only address the ones xwh did and nothing more.)
I agree that a BPD will often put the child in the middle already. I feel it my responsibility, then, to try to move the child out from the middle, to the extent possible. That means, if xwh tries to use the child to send messages, I don't discuss the message with the child, but empower the child for next time to tell Dad that he/she isn't comfortable passing messages and ask that Dad take that to Mom directly instead. It is up to you, with gauging the importance of the matter, if you then also go directly to stbxwh with the response to the message sent via the child, or wait for the direct contact from stbxwh to happen first instead. I have been known to tell the child that if dad wants to discuss that with me, he knows how to contact me/has my number/email, and then to drop it/change the subject. (Caveat: I don't tell the child to say that to xwh, only in the form of reminding the child that is the case, which then frees the child from the messenger duties, since they know the two parents do have ways to communicate directly instead.)
One thing about those texts from stbxwh, the ones you feel he may be trying to use to document: crickets is best in most cases, but if you feel he may be trying to document, to make some sort of case with them, then the best response I have heard is to simply state that you always strive to do what is best for the child, or to handle these things in a manner that is best for the child - and then drop it, or, if necessary, rinse and repeat, no matter what stbxwh says back. You could even thank him for the reminder that neither of you are to speak with the child about X matter, noting you know that is best for the child and strive to do the best thing for him/her.... In a way, it is not directly responding to the specific accusation, but it is addressing the overall issue, and speaks to the one big overall issue you feel he is trying to hit you with, with documenting. (It takes away his ammo - he's not going to like it, but, then, you don't like what he said to you in the first place either....)
On the bringing the OW around, I get how you feel disrespected, as I want mow to completely disappear, but at least she was down the street - another poster here at SI had the stbxws bring the AP into the driveway, had AP screaming at the poster, and had to call police each time, just to, eventually, bring back peace. Some others have had PO against the AP, to keep them away completely, as the stbxws would bring them, just to get a rise out of the one who posted it here. I know it isn't how you want it, but consider that, warped as it may be, stbxwh may feel he is honoring keeping mow away, since she's not on the property, or even just at the bottom of the driveway. (And it is better for the child if you don't show angst at this; if you must protest with stbxwh, do it privately, in writing, or, better, through your lawyer, and don't let the kids see. If you react in person, you have to know stbxwh is going to speak ill of you to the child for it as they drive away....) No, I don't like accepting "crumbs", but sometimes, it is all they have to give.... They simply aren't able to give the whole thing.
Really, the bigger problem there, and where your focus should be (see, he has you distracted by having the ow there, when this is what should upset you the most...), is that he brought the child back late, and that it was very late past the return time. I hope you documented and gave that info to your lawyer. Also, it may be worth asking your lawyer if you should call the police next time he is late, for a report to be made, and at what point it should be done. (Five minutes is not the time to do it, obviously, but should it be that 30 minutes passed, or an hour, or what? How does that advice change if he made contact about going to be tardy? Also, if he has made any contact to tell you of the tardiness, make sure it is in writing, and respond to ask he arrive as soon as is safely possible now, and, in the future, bring the child by X time, as noted in the order as the return time.)
In the end, I think one thing that helps when dealing with this personality is to continue the idea of it being "all business" - much is said here on SI as thinking of dealing with the D process as being "business", but I think that continues on for us. It isn't at all easy, as they know where your buttons are and how to push them just right, but if you can keep the emotions at bay and behave in a business-like, professional manner toward him and all his antics, it will help you greatly. It has me, at least.... That's not to say I haven't screamed into my pillow, or cried in the shower, or vented to IC or a friend in private, but I have done my best to show zero emotion to xwh at all, and the less he feels he is able to upset me, the less he seems to try, now - it took a bit of time and we went through some escalation in attempts by him, but he now at least presents a front of civility in all direct communications. (And he had gotten quite nasty at times, too, previously....) That in no way is to say his behaviors have completely done a 180, as he still does stupid stuff to/with the kids, but I only handle that by assisting the kids with handling it, empowering them to deal in a healthy manner, and say nothing to xwh directly. (It does help that my kids are older, but your will grow older as well, and get there, too.)
I hope this has helped in some way - take what does, discard the rest.... and hang in there, as gradually there will be less and less contact as the kids get older, which means more and more opportunity for you to have peace in your life again.