my favorite (conspiracy) theory is that he "moved on" to the development of Namecoin
Definitely not, because technically, NameCoin didn't follow Satoshi's description:
Instead of fragmentation, networks share and augment each other's total CPU power. This would solve the problem that if there are multiple networks, they are a danger to each other if the available CPU power gangs up on one. Instead, all networks in the world would share combined CPU power, increasing the total strength. It would make it easier for small networks to get started by tapping into a ready base of miners.
Read it sentence by sentence, and compare NameCoin's difficulty with Bitcoin's difficulty. In case of NameCoin, they have their own difficulty. They trace only 80-byte headers, which are submitted to their network. Which means, that you can do 51% attack on NameCoin, even if you can't do 51% attack on Bitcoin, with the same hashrate. Which means, that it is strictly incompatible with Satoshi's description, so it is very unlikely, that he was directly involved in creating this altcoin.
Note that in practice, no network I am aware of, can trace all block headers, and compute the global difficulty correctly. Even Bitcoin traces only its own difficulty, which is why copycat-altcoins can be 51% attacked, because the code has no assumptions, that there could be a stronger, but invalid chain of headers elsewhere. And there is no code to detect that situation, which is why you can 51% attack BCH, BSV, and other chains. If they would truly believe in Proof of Work, then they would at least trace the heaviest chain, and form appropriate consensus rules for that kind of situation.