Single-topology IS-IS with the transition feature is the correct design when IPv6 is being added to an existing IPv4 IS-IS network and both topologies must match exactly. In single-topology mode, IS-IS calculates one topology and uses it for both IPv4 and IPv6, which fits the requirement that the IPv4 and IPv6 paths and router levels remain the same on each interface. Cisco documentation notes that single-topology IS-IS IPv6 requires routers to run the same set of address families for adjacency consistency; during migration, transition support allows the network to operate while IPv6 is introduced gradually. Multi-topology IS-IS is used when IPv4 and IPv6 should be allowed to follow different topologies or when not all routers support the same address families. The question explicitly says the topologies will match exactly, so multi-topology is unnecessary. Single topology without the transition feature is risky during phased migration because routers that do not yet run both address families may fail adjacency checks or create blackholing. Reference topics:
IS-IS for IPv6, single-topology IS-IS, transition mode, address-family consistency, IPv6 migration.

Multicast traffic is transported in the overlay, in the EID space, for both wired and wireless clients.
https://www.ciscolive.com/c/dam/r/ciscolive/us/docs/2018/pdf/BRKEWN-2020.pdf