You need to get some legal advice asap!!
Also Lh86 my first thought is concern for your safety & that of your children. From what you have described he sounds like a man who is feeling very angry that he is 'losing control' (of the relationship / you etc). This point in a relationship breakup is the most dangerous time in terms of your physical safety. He has already shown that he is willing to go to some pretty extreme lengths to keep tabs on you & not lose 'control' of his children.,, & he is willing to use them as pawns / weapons against you (if the abuse stories are lies).
do not speak to the children about this / ask them questions about what dad might've talked to them about / coached them to say (if anything) as then you could be seen as coaching them too if it came down to legal arguments.
I would also speak with a solocitor with regards to their thoughts on contact.... given what you have said (his violent & additive history & the current allegations) i think you should absolutely be doing supervised contact so someone can monitor his behaviour & conversations with the children & ensure this remains appropriate. Many towns have centers which run supervised contact visits.... A solicitor or your local children's social work team should be able to point you in the right direction there.
obviously you have not been able to share your whole story here but please do find a solicitor, tell them everything, consider all the above & also discuss whether you need to consider a no contact order for yourself re: this guy (so he cant come near you). all of this is a very very delicate situation so you shouldn't go ahead with any actions until you have spoken to a solicitor, told them the whole story & then they can advise you.... You have to balance up the options... Keeping you & the children safe vs not aggravating your ex's disposition any more than necessary.
hope all that makes sense!
keep safe & don't try & manage this alone