I think HDMI-CEC is just fundamentally broken (at least in all versions of the spec up to now.) I've played around with hypothetical workarounds, and I suspect so have many device manufacturers, and it seems like there's just no way to "stabilize" an HDMI-CEC network into behaving correctly while there's at least one bad actor on it, if you can't directly modify the firmware of that bad-actor device. You can't do it from a peer device; you can't do it from a "man in the middle" like a switch; and you can't even do it from the TV.

Why? Because the HDMI-CEC bus protocol, as it stands today, has no separation of responsibilities/phases between "request" and "response" messages, or between "command" and "event" messages.

Imagine how you or any other reasonable person would design HDMI-CEC. There'd be one "leader" device (probably the TV) whose opinion on the current active source / signal-path is canonical for the whole bus. Other devices would send command messages to this leader device to get it to change the active source. The leader could ACK or NACK these command messages. If the leader accepts the change, the leader would broadcast an event message declaring to the bus what the new canonical active source is. And if a device misses that message, it could also query the leader device to find out the active source.

But that's not how any of this works. Instead, at any time, any device on the bus can just outright say "I'm the active source now." And every other device on the bus that receives this packet, must then update their own internal HDMI-CEC state-machines to consider that device to be active. If something (the TV, say) disagrees with this assertion, it can't just ignore the packet, because everything else on the bus won't have ignored it. So even if the TV had logic telling it to stay on source 1.0.0.0 rather than switching to 3.1.0.0, the soundbar (3.0.0.0) connected to said TV might have switched from playing the eARC audio stream from the TV to playing audio directly from its downstream 3.1.0.0. And 1.0.0.0 might have gone to sleep in response to seeing the message that 3.1.0.0 is now active. Etc.

The only legal thing a device that wants to "prevent another device from making itself the active source" can do, under HDMI-CEC, is to steal focus right back to itself (or, if it's a TV, assign focus right back to the source it remembers.) Which means everything on a bus sees the active source flapping back and forth.

Which is where all the stupid "your TV takes 30 seconds to turn on because it's flapping between three inputs" stuff comes from.