You are being MASSIVELY unreasonable.
Jealous, possessive and controlling, NOT ONLY over his contact with a FRIEND (if he'd EVER been truly interested in her or her in him they would have at least attempted a full relationship) which you have no right to dictate to him about but ALSO your possessiveness over her commenting about his daughter - when YOU have only been with him a year!
At this point you should barely know his daughter, being nothing more to her than "daddy's friend" and you NEVER have the right to dictate who his friends are. Plenty of us stay friends with ex partners/fwb/ons, as long as you have no REAL reason to suspect he's cheating on you with them - which you DON'T!
"when does a FWB cease to be so?" When they stop having sex which they clearly have.
"and if a new (committed) partner would like the above to be made defined and clear, surely it’s OK for them to need that boundary?"
It IS clear, but if your partner wants to stay in contact with his FRIEND then you have no right to demand he doesn't.
You were lucky he agreed to do what he did but quite honestly I think he's enabling your jealousy and insecurity.
"And I really don’t understand the FWB stuff at all." To be perfectly honest that suggests to me you aren't right for each other. You have different views on relationships, even though yours is not a Fwb relationship, that you can't understand them, something he clearly does understand and has found suited him at the time, then you're likely to disagree on other aspects of relationships too.
"I think some posters really should think hard how they would be feeling in my shoes" but YOU are unwilling to consider HIS feelings, someone you claim to care about!
"Some women are just nasty and have no moral compass. She is one of them" oh ffs GROW UP! She's done nothing wrong!
HER morality is none of your business!
I have been cheated on, I've also been in fwb relationships - the two are NOT necessarily connected.
If you trusted HIM this would all be a non issue.
You clearly don't and you also clearly have insecurity and jealousy issues that you should address ASAP.
You are going to drive this man away with your nonsense and if you won't tolerate "any grey areas" you are going to end up lonely and bitter. Life is FULL of grey areas, people always have grey areas.
You were married a long time you really should know this - did ex husband cheat?