I hope things start looking up for you. I am in exactly the same position although with no option to leave the country! Its sickening, gross and so frustrating that the RP has no say over what happens when the dcs are with the father.
I have to hand my dcs over in a few weeks so he can play happy families and pretend devoted dad whilst he gets married(slam bang ty mam) to his rebound he found after a month of us being apart. The dcs are bridesmaids/pageboys etc, it makes me want to vomit thinking about it.
In my head Ill never dignify her with a name or title, in my head(and my phone contacts!) shes the rebound and will be dcs dads gf, not wife, in my eyes you dont walk into someone elses life and take their partner or v recently exp when the exp is less than 6months postnatal with his youngest child and then expect everyone to see you as the wife less than a yr later.
What she did was unforgivable, he is obviously to blame too of course but really, who shacks up with a recently split man with several kids when they also had a child themselves?
I also get your feelings of losing years, i spent my latter teens and majority of my twenties with my exp, feels like 10yrs down the pan.
What I find helps is to think it probably wont last, lets face it, rebounds rarely do, especially in my case where theyve ran into their relationship without really knowing the other person. Rebounds happen because the other persons trying very hard to forget/get over the ex, not a great recipe for a lasting relationship!
I hope your lawyer helps gets you relocated-thats something i personally wouldnt discuss with him (there are court orders(in uk anyway) nrps can get put in place to stop the ex leaving the country with their dcs) concentrate on yourself, try not to think of that arsehole because he wont be thinking about you.