YANBU OP.
If you look at it like this, then what he is asking is unfair.
The OP and her DH both receive an identical salary and identical dividends from the business.
They have separate accounts for their identical salaries, out of which come joint expenses, but to all intents and purposes they are jointly and equally responsible for mortgage and school fees etc.
DD7 is going to private school but so did DD18, so the fact that the school fees come from OP's salary and account are neither here nor there. She's not favouring her DD with private school while living off her DH's salary and depriving his elder daughter. She has suggested a joint account which her DH has refused to consider.
The OP and her DH also have separate savings, which are theirs to do with as they wish.
He wants to use his savings to fund DD18 at university, which is his right if he wants to do so.
However he also wants to use the OP's savings to fund DD18, savings which she had earmarked for a separate purpose, namely paying off part of the mortgage and in lieu of a pension.
And it's not his right to use the OP's savings if she doesn't want them to be used.
The OP is not denying DD18 something which she then plans to give to DD7.
And there are other, more sensible options open to DD18 to fund her university place.
However the big warning bell is the tantrum he has thrown about this and his childish threats to liquidize their company if he can't have his own way. If this is the way he reacts to not getting his own way then you are probably going to need that money OP, because neither a marriage nor a business partnership can survive for long when threats like this are being bandied about for any length of time. Ultimatums have a way of damaging even the strongest relationships.