I guess you need gretap (Ethernet GRE tunnel over IPv4). The gretap on OpenWrt router 1 should be connected to the wan network. And gretap on OpenWrt router 2 should be connected to one of the lan ports which you have moved to a separate vlan in the router's vlan switch.
If you’re going over the open Internet, be aware that GRE doesn’t provide encryption. For “as if you are there” you typically need a Layer 2 bridge. Simple tunneling approaches have MTU issues. VPNs that can provide Layer 2 tunneling are preferred by many.
Layer-2 GRE tunnels allow you to have the same VLAN in multiple locations (separated by a Layer-3 network) and be connected. The forwarding method for a Layer-2 GRE tunnel is bridging.
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"House 2 Device" need to simulate that is in "House 1" to be able to join the multicast (video streaming). This is the only device that send/receive within the tunnel. Rest of devices and traffic in "House 2" are independent.
Yes I think running GRETAP over a routed vpn is a good idea. But I prefer wireguard instead of openvpn. (And I use GRE over wireguard as a way to access the Internet without adding 0.0.0.0/0 to allowedips.)