Quick point about marriage tax benefits: it’s a myth. Most taxes are the same, and any differences make a married couple *worse* off for being treated as a single unit financially. I warned my dad about this, but he went and got married anyway, and has found that he and his wife have both lost their widows pensions. Any rights that you gain from marriage can be gained separately by any appropriate legally binding contract you draw up. Most of these aren’t necessary for us now, relating to children and property etc. Certainly, not marrying allows you to pick and choose the benefits you want, when they apply, without getting them all in a great big taxful lump.
There are no rules about who your next of kin can be, you can simply announce it to the hospital on arrival or keep a card in your wallet in case of unconsciousness. No adult can consent to medical treatment for another adult, in any case.
I’ll let Dan explain the rest. It made me laugh that you think of Dan being injured rather than me, it’s by far the most likely way around!