You can bop her on the head at some future point in time, when your relationship is re established!
She’ll probably realise she deserves a bop herself one day (not until 24/25 tho).
Whatever the reason for X being present, just say yes.
If you say no it will give your DD an excuse to cancel the meet up and give X the opportunity to paint you as the bad guy again.
It’s possible that X’s behaviour up til now is driven by X genuinely believing you and DH are a pair of evil, controlling, transphobes rather than a family negotiating some difficult times after illness/bereavement and that coinciding with your DD’s hitting the often-difficult teenage years.
If it were me I would try and act like I was meeting my DC’s romantic interest in completely normal circumstances- a warm smile, a handshake, a hot chocolate at Costa and cake in a paper bag to take home.
Show X and your DD that you are a fully-rounded, caring human (not a Disney villain!) who loves both your DC and wants the best for them as they move towards their adult lives.
You could even thank X for giving your DD the confidence to get a part time job!
If X feels like X can trust you, X may well become less vigilant for perceived slights and signs of transphobia and relax enough that DD will feel able to see you more freely (and perhaps eventually, to move back home whilst continuing to date X).
If you refuse to meet X then you will just creat an adversarial situation where your DD is forced to choose between you and as X and X’s home currently seems like the safe option, it’s likely you’ll be the one who is rejected.
Maybe this whole thing is a massive misunderstanding caused by everyone’s projected emotional baggage and your DD feeling excluded while you’ve been busy with grief and your DH has been hiding at work?
X has no doubt been getting the same ‘your parents are boomers who hate you and are ruining your life’ messaging from the internet as your DD has - that X’s parents have managed to negotiate this phenomenon and keep their teen in the house might be more to do with them not having the really big life disruption events that your family has had?
I’m sure my DsD’s Gender stuff really took off as a result of her little sister’s cancer - the family was looking the other way and she felt alienated and lonely, this coincided with puberty and suddenly looking like a woman and getting cat called and she disappeared inside the trans identity as a combo of protective shell & maladaptive expression of her need for connection with her dad.
Seems to me that your daughter needs her fam (who have been dealing with some massive, earth shattering, unavoidable situations) and pushing you away is a similar protective shell. She doesn’t risk rejection/not getting her emotional needs met if she runs off and shelters inside another family’s life, a family who hasn’t got a grieving mum and a workaholic dad.
Apologies if that feels like an overstep - I’m not a professional and much of what I say here is a projection of my own experiences, so do keep that in mind.
Rooting for you!