Custom pages with plugin

I’m developing some plugin where I would like to enable custom pages. In my case some custom page would contain a form like contact form (not literally). When user will fill out this form and send it, there should be the next step which will require more information. Lets say that the first page with form would be located at www.domain.tld/custom-page/ and after successful form submission, the user should be redirected to www.domain.tld/custom-page/second. Template with HTML elements and PHP code should be also custom.

Passing and retrieving query vars in wordpress

I have two authors pages, one displays about 5 posts. Then I’m trying to setup another page that will be all of their posts. I have created a template called moreauthorposts.php and I’m trying to pass the author variable to this page. Problem is if i pass domain.com/more-author-posts?author=johndoe it gets stripped out. How can I retrieve this value? Is this even possible in wordpress? I know WP Rewrite is jacking my URL structure somehow I’m just not sure.