Running two Pi-Holes at the same time / Replacement + Setup

Yes to the two first bullet items.
The third (pi.hole) could be addressed by configuring PIHOLE_PTR (available since FTL v5.10.1) on at least one of the machines, but wouldn't lead to immediate issues if left as is, apart from ambiguity when accessing Pi-hole's UI from the new device while both RPIs are active.

Yes, plus the equivalent for DHCPv6 and RAs, provided your router supports those IPv6 related configuration options.