Multicast capability has been quite patchy with the 8030P series. Some firmware releases had it, some didn't. I'd recommend you replace that kit with something from a vendor who cares about...

That is in fact the general idea in adding WiFi repeaters. Yes of course do you use the same SSID - and make sure you set them up to different channels such that you get good range overlap without...

... wherein "repair" is actually re-pair, i.e. restore the lost pairing of the adapters.

Sure you can, just follow the instructions to pair an extra adapter into an existing PLC network.

The only thing you can do there is improve your power wiring, or run a LAN cable as far as you can, e.g. to the stable. Run a PLC stretch from behind that extra fusebox to the shed. When installing...

It's probably that extra separation/fusebox that ruins the signal, but even if it wouldn't, 200 meters out in the open is literally stretching the maximum range for PLC. If this is three-wire...