I feel worry for you when I read through your writings about the situation you are living with. Especially with the difficult, traumatic events that you have coped with.
You talk about falling very quickly in love, of feeling secure with your boyfriend. My feeling on these sorts of very sudden feelings for strangers is that they say more about us then about the people. Maybe he reminds you of somebody who was lovable an who did give you security. His posture, way of moving, aftershave, dress sense - any of those could trick your brain into associating him with love and security. The problem is that our brains can be tricked like that. The reality of the new person may not match the reality of the person they are being associate with. So - in your shoes I'd be mistrustful of these feelings.
And then there are your boyfriend's habitual thought patterns. He has cheated (with more than just you?) on his wife. He likely has lots of stories in his mind that justify to himself why it's okay for him to behave in such a hurtful way toward somebody he loves. I suspect that these stories and thought patterns and the behaviours that go alone with them are by now comfortable habits for him - things he will go back to regularly unless he recognises them as such and deliberately takes action and works on stopping them.
And now there is the pair of you. Now he's living with you and you are in the role of his wife - you are the partner he can be open about being with, you are the one he comes home to the one he has to lie to. This coupled with the fact that he seems from your writing to have taken no action or ownership of his habitual cheating thoughts and behaviours makes me think that you can expect to be lied to and cheated on for as long as you are with him.
I doubt if you being happy with poly and clear with him that he is able to have sex and romance with other women will make any difference to the way he behaves. Having sex with others may not be cheating in the relationship you share with him but until they are addressed, he will still need an outlet for the habits he has created in his thoughts and action around settled relationships.
I think that he will still need to hide things from his spouse, lie about things and refuse to discuss what he's up to (all of which he has already done to you). I would bet money that if you ever ask him for any kind of boundaries around sex and romance (going slow, avoiding certain sex acts for a time with other partners, periods of monogamy while you get sorted etc) he will break those and lie to you about it.
I know it's easy to say because I don't love your boyfriend and am not in your position but I would be thinking very seriously about leaving / asking him to leave.
I don't your heart is safe with him. I think that the very early, deep feelings of love and safety you felt were projected onto him by your mind and body but that he is not worthy of them.
I wish you luck and strength.