Suggesting reviewers for a very sectorial double-blind submission
I would suggest (1), with a tweak. There is no need to mention that these reviewers may have seen it at a conference. As you said, it does not constitute a conflict of interest. That is the only reason you should mention additional information about a reviewer.

Let's be clear, there is nothing illegal about a reviewer having come across your work earlier. It may break blindness, but then, blindness is often broken in double blind reviews for various reasons (small communities, typical writing style, figure formats etc.). This does not really compromise the review. The only exception I see is if the reviewer is a petty competitor, who would use his prior knowledge to rule against your paper.

If you feel that any of these reviewers may be biased against the work, don't suggest them as preferred reviewers, regardless of their standing.