Shammy1b, 'kicking off' is not the appropriate response right now. The fact remains that the OP will have no control over who sees her kids when they eventually split, just as your Ex has no control over which of your friends, male or female, you choose to let interact with your children. Of course, the decent thing to do would be to let one's ex partner know who their children will be seeing at any given time, but there has been little in the way of decent or rational behaviour on both the OP or her partner's part in this sorry situation so far.
OP, pick your battles.
You are in a precarious situation right now. As frustrating as it is, you need to focus on the important things first. Who sees your children on contact days is a fight for another time. As long as there are no safeguarding issues, and you currently have no cause for concern about that at this time, then your STBX can choose to see who he wants to, unfortunately.
As for your 12 year old, he needs to come home now, and you need to make it clear that none of this is his fault. You need to let him go on his trip so that he can get away from the situation for a while. I agree with a PP that he should not be with his paternal family when the shit hits the fan and you move out, because they may make it very difficult for you.
Your recent posts about him are not pleasant to read. You show an open hostility to him - as I said previously it's like listening to bickering siblings rather than a parent/child dynamic. You need to re-evaluate your entire parenting ethos, and help is available to you if you look for it.
With regards to the arsewipe, keep a cool head. As has been stated several times, don't engage, and don't get drawn in to the games. And stop telling him your plans. You will not hurt him. You will only sabotage your own escape route. Get your affairs in order first and foremost.
What have you done about getting your child back in education? Have you gathered all of your important documentation and financial information? Have you started to look through your posessions to see what you want to bring and what you can leave behind?
Start to be proactive OP.