The flows are the easy part, the stocks are much harder.
1 partner enters the marriage with a $500K paid off house, the other enters with nothing. They divorce 20 years later and the house is appraised at $2.5M, should:
How does this answer change if the house can be sold vs can’t be sold? How does this answer change if partner 1 comes into the marriage with a partially paid off mortgage and both contribute to mortgage payments afterwards? How does it change if the house is underwater instead?
The problem is all of the above models are legitimate models of how to split ownership, but they’re incompatible views. So you need to reach alignment at the start of the relationship over which model to use, otherwise what could have been an amicable parting ends in bitter recrimination.
The general rule is that property owned prior to a marriage remains the separate property of that owner. So the answer to your first question is generally A.
The answers to your other questions will depend on whether the couple is in a marital property state or a community property state, because the default rules regarding marital income are different.
Yet if you take that view then by sharing in the mortgage payments partner A may have become landlord to partner B and renter’s protection might apply to partner B.
If three was a mortgage on the property the payments on the principal are presumably community property, and the appreciation of that post of the value.
1 partner enters the marriage with a $500K paid off house, the other enters with nothing. They divorce 20 years later and the house is appraised at $2.5M, should:
A: partner 1 receive $2.5M, partner 2 receive $0?
B: partner 1 receive $1.25M, partner 2 receive $1.25M?
C: partner 1 receive $1.5M, partner 2 receive $1M? D: other?
How does this answer change if the house can be sold vs can’t be sold? How does this answer change if partner 1 comes into the marriage with a partially paid off mortgage and both contribute to mortgage payments afterwards? How does it change if the house is underwater instead?
The problem is all of the above models are legitimate models of how to split ownership, but they’re incompatible views. So you need to reach alignment at the start of the relationship over which model to use, otherwise what could have been an amicable parting ends in bitter recrimination.