Hi WorldWithoutGrudges - and thanks for sharing with us !
Not sure where you are from and that might have some bearing on what choices you consider from the legal side.
Your questions are very valid ones and I give you big kudos for thinking ahead logically into the future.
Although other may feel differently, my outlook on 'marriage' has always been that it's little more than any other legal contract. And because of the old historical model embraced by government and the court systems there is still total bias towards the two person household.
So you have to approach things logically in regards to the legal implications - be they tax status, insurance, parental rights etc.
One concept that seems to be gaining in popularity is the formation of LLCs. Along with this can be legally binding contracts spelling out rights & responsibilities for things like parental issues, survivorship rights, property issues etc. Unfortunately this can't cover ALL the bases at this time due to current government bias but it can probably cover 95% ! You may want to study this before you even consider classical 'marriage'.
Maybe someone here who has gone further down that path will have more insights.
Good luck !