OP, the time has come to act - IMMEDIATELY. I know it's hard but break through being all British and not wanting to rock the boat, upset people, etc., and take control. She is relying on that British reserve to not say anything when she does stuff like go to parents evening, etc. Get on the front foot: contact a solicitor, contact your HV to talk to her about everything you've discussed with us, hell, even contact SS yourself about concerns that someone this unhinged is having so much contact with your DC.
Luckily for you, the way she has gone about all this will not actually make it hard to prove she must have no contact with your Dc and for professionals to see her crazed machinations for what they really are. These are the factors you should concentrate in proving her dangerous:
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The tattoo. This is out and out insane. You just don't do stuff like that. It makes her look really unbalanced and overly possessive of someone else's children (because she is). If you have to go to court, etc., judges are notoriously stuff and generally take a dim view of tattooes in general, never mind something crazy like this.
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Parents evening and taking the kids out of school, etc. She has behaved bizarrely and badly and in a totally overstepping the mark way with that particular teacher and the school in general. That teacher will no doubt be more than happy to recount how weird and aggressive she was about another woman's children she's trying to pretend to the world are hers. Also, the school is not going to look good having played along with her and will want to make sure outside parties know all the blame is with her being nutty.
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Medical appointments and the wheat allergy shit: I would not be surprised if she has passed herself off as the children's mother. The GP's surgery and Dentist's surgery could get in trouble for letting her take them for appointments that the residential parent, ie you, has no knowledge of nor given permission for, and again, will be keen to not be in trouble or viewed dimly and so state that she lied and bullshitted, etc.
You also need to get access to their medical records to see exactly what was said and done at these appointments: she could have Munchausen's By Proxy and be taking them because she has deluded herself thT there are things wrong with her, or she could be taking them as a calculated step to try to plant seeds of doubt about your parenting via official records, ie, saying she's worried about their teeth because you feed them crap, for example, lying that she thinks you've hurt them to the Dr, say, the obsession with the wheat allergy, and so on. Or it could be a combo of both those things. Whatever - you need to turn her scheming against her and use it to prove that she is obsessive, deluded, deceitful, untrustworthy, oversteps the mark in grand fashion and above all, that doing all this stuff is actually psychologically (and perhaps physically, if she's got them unwanted treatment via lying) abusive.
- SIL and others: thank god for SIL! Fruitloop is very obviously mentally ill and others have clearly seen and experienced her ways. Get these people on board; they will want some sort of 'justice' or closure for the hurt she will have caused them. Get it documented officially that she has been lying to people about bruises and the like. Make sure it's made officially recognised that people who know her can clearly see she is working on a game plan and aren't fooled for a minute.
A word of warning: do this before they get married. When she is his wife, outside agencies will get that word 'wife' stuck in their head and, because some aspects of her behaviour border on the just-about-acceptable for someone who is legally their stepmother, that could unconsciously cloud their judgement when viewing all this shit she did when she was just your Ex's girlfriend (ie viewing her past actions through a 'wife' filter, when she wasn't the wife at the time). It's a nuance, but it's a critical one.
BTW, Your Ex sounds an ineffectual arsehole who is just making things loads worse by being lazy and choosing an easy life for himself over the needs of his children AND, in fact, his partner, who quite obviously needs immediate and intense mental health treatment.
Good luck! This is so chilling that think that people couldn't handle this if it was a film or book. She needs help, but more i portantly, your Dc need to get away from her. Presenting herself as their other mother or even as the more important and 'real' mother is down and out abusive. I reckon your Dc will need counselling to make sense of this all, it is beyond fucked-up, wow.