I definitely agree on the complexity of our tax / govt financial and legal codes, and the problem that poses. The solution there is obvious too, and has been needed for decades anyway: simplification. Of course that's held up by political gridlock.
The easiest way to resolve the child situation would be to keep custody to two parents as it is today. Any children that exist in the N=5 marriage, would be biologically between two people. So the only thing that would be changing is marriage itself, not child custody laws (which today are not primarily built on marriage anyway, we can obviously marry other people that have pre-existing children without adopting those children as our own; and we can obviously have children outside of marriage).
The easiest way to resolve the child situation would be to keep custody to two parents as it is today. Any children that exist in the N=5 marriage, would be biologically between two people. So the only thing that would be changing is marriage itself, not child custody laws (which today are not primarily built on marriage anyway, we can obviously marry other people that have pre-existing children without adopting those children as our own; and we can obviously have children outside of marriage).