> Unfortunately, my ISP hands out a single /48 prefix, so I will be unable to test multiple prefixes.

Me too, but there is probably some ISP out there that does.

> dhcp6c is only used for IA_PD, so I suppose lifetimes for IA_NA can be ignored in the router's firmware.

It is used for IA_NA (see [release/src/router/rc/udhcpc.c:760](https://github.com/RMerl/asuswrt-merlin/blob/master/release/src/router/rc/udhcpc.c#L760)) when the RA managed flag is set. Currently none of the NA info is passed to the state script, though.