How effectively can they share data domestically during blackouts?

Carrier pigeons.

And hamsters.

Don't underestimate the hamsters.

Especially ones in a station wagon full of tapes hurtling down the highway.

But seriously?

There are mesh networks, but even so IPv4 isn't down, and people can share files p2p.

Theoretically this is true, but in practice it's not. Most p2p services rely on the global internet in some way. The BitTorrent DHT, for example, is unlikely so self-heal in the event of a completely inaccessible global internet.

Things like HolePunch have a lot of potential here, but you'd need an Iran-only DHT, and it's just not deployed at scale.

but, regrettably, at a trickle