Agree - as you say you wouldn't want to upheave him again now. But it leaves bad feeling when you adopt on trust following an assurance that there are no issues (or at least that no issues were revealed). And it has cost you in vet bills and worry.
I am not sure what else a head tilt could be other than a dislocation - and assume you couldn't tell that without x rays. It sounds like you think he is in pain - hence the metacam presumably. Has he bitten at all? That can be a sign of pain. On the other hand he sounds to be hamstering along and using his wheel.
I don't think a short course of metacam would do any harm but if he's on antibiotics, I'd give probiotics for a while after he finishes them. Dr S from rat rations is one Cypher used to use - I've used benebac for small animals - available online in the Uk and recommended by vets in the US. As antibiotics can affect their gut lining quite badly. A short course again should be fine but it can help to have the hamster specific probiotics afterwards.
Agree about giving a half dose of metacam and seeing if it makes much difference. Although that could be tricky! The dose must be tiny for a robo even at full dose
