Learn the difference between mixpanel and google analytics. How both of them track users and how long a user is stored in cookies.
Its not possible to track any url that is not linked with your google analytics account. For more details read this article.
Learn how to use google tag manager events to track pop ups by pushing an event within data layer and set up trigger using that event.
Here is how to find the landing page and conversion path in google analytics. Find more related resources about conversion path and landing pages.
Google analytics user flow reports are helpful in finding where the user went after a specific page. Read more here.
Learn how to enable enhanced link attribution in google tag manager. Get other resources for learning about link attribution in GTM.
Here is how to filter your data in order to exclude returning visitors for getting the correct bounce rate. 4 ways explained to get this done.
Yes, if you have implemented google tag manager on your site there is no need of google analytics id. Because Google Tag Manager is capable of doing more things for you and it can