I agree with everyone saying "if you're not ready to get married, you're not ready to buy a house together." I mean, really. You say you're "Laying out all the foreseeable contingencies, even though unforeseeable would undoubtedly arise, is what I intend." And in the next sentence, "Dealing with any other issues fairly and amicably seems attainable." Well, guess what? A foreseeable contingency is that, for whatever reason, in the future, you can't actually work out an issue fairly and amicably. And you know what's really bad for your finances? Lawyer bills to hash out an "unamicable" contract dispute.
One (more complicated) option to one person buying and the other paying rent is this:
One person buys.
Other person pays half the mortgage every month.
Have a signed document that, if the relationship sours, person 2 gets bought out by person 1. In other words, person 2 gets all the equity that they've put into the house.
Person 1 can take out a home equity loan in such a circumstance.
In this way, you're both accumulating equity in the house, but only one person officially owns it.
Disclaimer: I'm not a lawyer, so I don't know if this would work.