Why would it be worthless? It’s still a valid, deliverable address to a real person (you). The only difference is the receiver knows which company sold their information AFAIK

I’m fairly certain you’re wrong about the “.” in an email address

Why would a scammer care if you figure out which 3rd party sold you out? I don’t think the risk is worth paying for another subscription.

Probably not.

The best advice I’ve heard is to use a variation of your email (assuming you use Gmail) on every site you sign up for that indicates that website. This would allow you to immediately know.

So what is a Gmail Plus address? Say you have an email address like billgates@gmail.com. If you append a “plus” sign to your email username, Gmail will ignore anything written between the + and @ sign in the email address and still deliver the message to the same mailbox.

More info