He's being unkind because you refuse to throw yourself prostrate at his feet, wailing that you'll wait for him to come back to you... that's why. He can't understand why such a prize as he does not have the power to keep you in thrall even now that he has thrown away his marriage.
You're getting on with your life beautifully, taking steps to cut around him, extricating yourself whilst still maintaining a detatched relationship with him for your childrens' sake. He can't understand how you can do this and that's why he's constantly trying to be wistful with you, talking about the past and your shared history. About his wretched wedding ring - he's talking about that to try to break your resolve - and you're not budging. Of course it's a low blow, he's losing his impact with you so he's trying harder, upping the ante. Many women might have faltered at the talk of the wedding ring - you didn't. That's some strength you have inside, VB.
I absolutely think your solicitor is right. Formal is the way to go. The more you talk to him about anything really, the more he will try to push your buttons.
He can't bear the thought of not having you on 'Team Thickwit' any longer... he's lost the respect of his parents, undoubtedly some of his/your friends will have changed their views of him - and the oh so romantic relationship with other woman has been imperceptibly changed (by your fab solicitor) from Romeo and Juliet to the all too mundane 'partner'. Ha ha! I expect the scales will be falling from her eyes pretty soon and perhaps they've already fallen from his. If he were truly in love, he'd be getting on with his own relationship, not focusing constantly on you... he doesn't sound like a man in love to me. He sounds like a weak and feeble man who has made his bed, doesn't want to lie on it - but must, because all other beds are closed to him.
He doesn't want to lose face because then he'll have to admit to himself that he was wrong all along and that he's lost everything. You know that he can lie to everybody, even those he professes to love and care for... one of these days he's going to have to wake up to the realisation of what he's lost and it's going to hit him like an express train.... and you'll need to stay detatched and collected, no emotion, even though he'll be trying to cling on to you.
He already knows that you will one day find happiness with a partner again; true happiness with somebody who loves you and will also love your children. You'll be ready to move on by then, your broken heart mended. He, on the other hand, will have nothing new... just a continuation of a relationship that has taken him with it, that he can't afford to jettison, because there isn't anything else on his horizon.
He's an angry man, VB, angry at his loss of control of you... you should be celebrating that because it's a heck of an achievement that you've extricated yourself with such dignity. Be proud of you and how far you've come, you really are an inspiration.