1) Through a cb subs plan, can i adjust the joomla template that the user in this plan will see?

No, not unless you've some sort of template manager extension that allows you to assign a Joomla template to a specific user id. In which case you'd be able to adjust that value using CBSubs, but CBSubs it self won't serve them a Joomla template based off subscription.

2) I need to have a menu item in cb menu where it will point to an external url but i need the user to insert the external url. So, i need a menu item (external url) that will be different to every cb user (like cb fields that take different values between cb users).

Dynamic menu URLs are not possible in Joomla unless you've some sort of extension that can hook into a menu and alter it dramatically before display. As far as I am aware there's no such extension available to do this. Your best approach is building a menu dynamically using CB Content and substitutions, but you'll need to supply all the HTML for the menu display.

3) In a cb subs plan, is there a way to give access and view to a specific article?

Yes, install the CBSubs Content integration then on Integrations > Content within your plan you can protect specific articles.

As with 2nd one is there a way to create a button leading to URL field and have a menu item linked to this field somehow?

Sorry, I don't understand the question. You want to link to a specific CB field? No that's not possible.