bitcoin solo mining that is used by 0.1% of bitcoin miners today.
It's not just for solo mining. Different pools also need to reach consensus among themselves. Your argument would be valid only if 99.9% of bitcoin hashrate came from a single pool.
Your protocol fails even in the very likely case that two pools mine two blocks with different merkle roots.
I don't see why this would be more problematic than the bitcoin network with mining pools.
Anyone can emit some work, as block header + work circuit, then miners decide on which work they want to mine, like they would choose a mining pool. If the work is invalid then the block is not going to be accepted by the network and they loose their work, so they need to have source for valid work with valid transactions and circuit that will match the protocol and will be accepted by other nodes.
Bitcoin protocol doesn't rules this at all, in theory nothing would prevent to have hash rate evenly distributed between 1 millions pools and the blockchain being in constant conflict. Miner choose large trusted pool because it maximize their reward and decrease the risk, the same logic should apply with this system.
It still require a form of miner id to distribute the work, and this miner distribution should be the same for the whole network, but then there can be different block headers proposition with different work circuit. As long as the final the proof contain enough information to confirm that the work is made and conform to the protocol, i don't see why this is more a problem than the current configuration with pooled mining.
Miners should have some sort of work-seed that they agree to work on, and all miners can check that the transactions are valid and match the merkle root, and that the work circuit is conform to protocol specification.
If the miner id is made with address/ip pairs, then ok one obvious problem is avoiding IP/address spamming. But if there is IP spamming, does all the IP lead to the same physical machine ? In that case that can be detected, and it doesn't even need a strong consensus because everyone can check this, and if IP are found to be clone of the same machine, they should be excluded from the miner pools. If it need a different physical machine or system for each, it's already not the same attack cost. And that would be the only garantee that the system give, you need to have a unique IP and address to be able to mine, and anyone who can provide that has equal chance to earn a reward, no matter how much cores or computational power it has.
If the decentralization barrier become about spamming address and IP for 1$ the billion, it's still not worst than 10 000$ of mining equipment and an hydroelectric dam.
The same principle still apply that even if miners provide an invalid block, proof of work, or that the circuit cannot be proved to fit with the protocol, then the block is rejected and miners wasted their work.
I'm not saying there is no problem with this approach, but i don't see a "fatal flaw" with it either.
You seem to have this stance that proof of work = bitcoin protocol, and if a solution doesn't follow the model then it's flawed, but that's what i would call dogmatism. Yes it's not the same model, it doesn't have the same characteristics and requirement, and it needs a good system to distribute the work, additionally to the pow, the proof for a valid block need to contain the miner circuit as well as the work itself, but is it really unsolvable , i don't know.
I'm just doing surface study for the moment, maybe there is something i don't see and i just take the problems one by one, and see if there is a really a brick wall to them, or in the end how resistant to byzantine fault it is, knowing that decentralized distributed system cannot keep consistency with 51% of bad nodes. That's same for bitcoin and any decentralized distributed system.
