Are all solidity oriented questions
on-topic by virtue of being about the programming language that
Yes, I think all questions about Solidity are on-topic as said by OP.
When this ESE (Ethereum Stack Exchange) graduates, if a relationship is set with stackoverflow, I believe that Solidity questions on SO can be migrated to ESE: Will it be possible to migrate questions between the trilogy and the Stack Exchange 2.0 sites?
Yes solidity is always considered on-topic, even if used in other context, since it's central part of the ethereum ecosystem.
In the future, if there are ecosystems using modified solidity implementations which are not backwards compatible with ethereum, we could reconsider if such questions are off topic.
But for now there is really no point in migrating solidity questions to stack overflow. Should we add solidity to the tour page?