WordPress.org

Do I have to change my templates in order to make the plugin working?

No. You do not have to change anything. Simply unzip, upload and activate the plugin.

What will happen with my old Pages' Title?

The default Title will be printed unless you add your custom heading

What will happen if I uninstall the plugin?

Again, the default Posts'/Pages' Titles will be printed

I would like to use the plugin to display a subtitle on my page (to display both the Title and WP Title 2 Heading). Is that possible with WP Title 2 and how?

Yes. In that case you have to change your theme templates:

<!-- this will print your original Title: -->
<h2><?php remove_filter('the_title','wptitle2_the_title',999); the_title(); ?></h2>
<!-- this will print WP Title 2 Heading: -->
<h3><?php add_filter('the_title','wptitle2_the_title',999); the_title(); ?></h3>

My original title does not appear in the navigation. Instead, WP Title 2 Heading is printed, or link is broken. What shall I do?

Have you got any special characters in your Heading? Try disabling HTML in WP Title 2 Heading field (this can be done in Options -> WP Title 2). If you prefer using HTML tags in the Heading field, enable HTML in plugin's options, but remember to properly encode all special characters (e.g. & => & , " => ")