OP, this stood out:
‘...he is planning for our 16 year old DD to work in the businesses cafe/office after her GCSEs alongside with this current "work daughter" so he has been encouraging their friendship and gone with dd to help her out once.’
He’s recruiting your daughter to legitimise and normalise the relationship he’s forming with this young woman. It’s a form of cover, and it’s a sick, sick thing to do to his own child.
If I were in your daughter’s position, I would be hating this. Dad forcing a friendship with this young woman, having to witness how he acts around her...all of it.
I would also be hoping that you would protect me.
So, you need to take matters into your own hands.
Tell him his behaviour is weirding out his own child.
Tell him it has to stop.
His behaviour is entirely inappropriate. The rides, the ‘work dad’ thing, the conversations on personal topics - he needs to shut that right down, for the young woman’s sake as much as yours and your daughter’s.
He’s a grown-ass man. No way he doesn’t understand about boundaries.
Even if it never went any further, it’s still totally unacceptable.
Sending strength and hugs, OP. This is an awful situation to be dealing with.