How to encourage an existing list to verify (double opt-in) & related questions

Hi! After watching a Klaviyo tutorial about the importance of double opt-in, I changed both my pop-up form list on the Shopify website and my list that comes from my Unbounce landing page to require double opt-in. (At least, I THINK I changed the Unbounce list - it looks good on the Klaviyo side, but perhaps the form on Unbounce needs to be tweaked?)

As a result, I have a couple of questions related to that.

1. I have two separate lists instead of a main one - my pop-up form / welcome newsletter flow list and my Unbounce landing page to give a freebie if they sign up list. From those, I created two more lists : unverified and subscribed. I’m noticing that people are still signing up (to one or the other) but not verifying because my unverified list keeps growing. I thought that the double-opt-in requirement would prevent unverified people from being added. How can I find where the error lies?
2. Before I watched the tutorial I had single opt-in. So my current number of unverified is over 400. My plan is to send them an option to verify / double opt-in and tell them they will be removed from my list if they don’t verify (in nicer language) along with a special offer. But how do I do that? How do I give people the option to verify if they are in an existing list? Is it a form that I need to include in a campaign?
3. And finally, once I have my true list of people who want to remain, how do I suppress all the other people who chose not to verify? I’ve been looking around and nothing leaps out at me for how to do that.
   
   Thank you so much in advance for your help.