OP,
I think you are asking the right questions in your 13.30 post. Your position, rights and so on will vary as your family circumstances and contribution evolve. You might want to be careful about agreeing to locking in a position that appears fair now but will not be fair as circumstances change.
It might appear 'fair' that your DPs financial contribution and superior earning is protected now when you don't have a job and don't have children.
Over time, you may end up earning more and contributing more than your DP depending on what field your PhD is in and the intelligence and skills you deploy. I know some quant PhDs who earn mid-6 figures.
You mentioned children - You may decide to have a career break to have and look after children and to 'support' your DPs career with his agreement or preference (as many posters on mums net appear to have done). Or you may decide to return to work. If you have a long career break and your DP's career progresses then you might find yourself at financial risk.
If you haven't already, have a look at some of the posts on other threads where women who have put their careers on hold to have and raise children with their DPs agreement have been left in a pickle when their DP has ring-fenced assets. They are left with impaired earning from a career break, difficulties getting full time work with their caring responsibilities and difficulties paying for a house.
Marriage will legally change your status though not necessarily give you all the protection you might want if you are not on the title deeds of your main residence/property. Children will change the balance of protections again.
It is possible your DPs father has both your differing interests equally in mind (though I suspect not based on a close reading of what appears to be a carefully worded email. Unlike some PPs, I recognise this is supposition on my part).
If your 5 year relationship continues then you will have these as ILs so I would be really interested in what is truly in his mind and ask him so you know where you are with them.
Good luck.