I don’t think I’m qualified to give you advice, but I personally wouldn’t risk disassembling lenses myself to clean them. There are certainly guides online, but lenses have complicated optics inside, and at worst you risk irreparable damage to it. If the spot is too big that it bothers you (which it seems so), you might want to seek professional help. For the time being, I think it’s best to ignore it, or fix it in post.

Theoretically, you could have a bot account that subscribes to every known instance in your instance. That would solve the search problem. I’m thinking that would also be the solution to how search engines would index the content on here.