Mnetters post here all the time on a range of subjects that might not align with your experiences and/or morals. OP is perfectly entitled to seek opinions and ask for others' experiences in relation to her life. Who are you to judge her for asking questions?
I find it disgusting that members have pointed out her name-change fail. She said she'd changed names to ensure her anonymity and the first thing some of you do is start throwing up her past identities. That is morally wrong. You should be ashamed.
So many of you assume that relationships borne from affairs will fail. Well, I can tell you that to create and endure a long-standing relationship resulting from 2 broken marriages (and children) takes herculean efforts requiring patience, compassion, humility and understanding.
Serious-minded people who leave unhappy marriages to be with someone else, face an incredible amount of trauma of their own. It isn't just the guilt (which is considerable), but the massive upheaval in their families, sometimes losing full-time custody of children, being ostracised, losing so many friends, sacrificing family gatherings, the financial loss, family not speaking to you, the shame of the circumstances of your union. So many things against you as a new couple. You have to have a very strong union to withstand that and get through it.
Everyone seems to think that the new couple, borne from leaving marriages, is all roses. It's not. You have to endure a lot of slights and exclusions.
If, having withstood all that, and having forged a loving relationship, possibly with children, do you really think that "marrying your mistress creates a vacancy?" No. It doesn't. Nobody sane would ever go through that hell again. Emphasis on the word 'sane'.
OP, what did your partner's ex write to you about?
People cheat. I know they do. But people rarely leave a long term relationship for someone else unless they are deadly serious about it. Don't mistake cheating for a serious life decision.
And be kind. OP is just another Mumsnetter who asked a question. Give her your respect by either leaving well enough alone or by contributing genuinely, and without bitterness.