I work in this area OP and unfortunately I think you’ve been majorly misadvised if you think 12.5% is fair. I think you also need to stop looking at what is ‘fair’ and ‘right’ morally and legally where you stand.
To clear up some simple legal points -
• The fact that you purchased the house at a reduced price due to family connection is irrelevant and will have no bearing on the eventual equity split that is awarded. As a PP has said, only if the discount was given by way of a deposit contribution that could be ringfenced could that be the case. Ultimately you have a joint mortgage and have contributed equally throughout, therefore are equally entitled to the equity.
• In the eyes of the law you only have 1 dependant, as does he. Your 17 yo doesn’t live with you and your 19 yo is an adult. Therefore you have equal dependant responsibility.
• Given that you only have 1 dependant, you are over housed in a 4 bed property. Consideration will be given to how much a 2 bed property would be in the school area, but likely would be very affordable for you given the amount of equity.
• He has as much right, and need, to a mortgaged property as you do as you have equal dependants.
• You have your dependant the majority of the time so are obviously entitled to support financially with that. That’s what child maintenance is for and he is fulfilling his obligation there. It has no bearing on the equity from the property.
Ultimately OP, if I was acting on behalf for your ExH I would absolutely be advising him to take it to court to force you to sell. If I was acting on behalf of you I would advise you to seriously try and see where you could raise the money to buy him out should you insist on staying in the property.
I will caveat all of this with - it is shit. Morally, it probably isn’t fair. But very little about divorce is ever morally fair. I would strongly recommend you go and see a decent family lawyer specialising in this area if you do want to fight it. Good luck.
Edited to add - the fact that he is currently paying rent on a property will be considered a joint expense until the divorce is finalised, as it will be argued he is being forced to do so due to your unwillingness to release the equity from the property to allow him to buy/pay down a mortgage.