V2 already has a module for making staff applications, it is called "Forms module" and it can be found in Nameless resources page.
Here is direct link: https://namelessmc.com/resources/resource/43-forms-module-for-200-pr7/
Considering Argon panel template hasn't been updated in over a year, it doesn't support new pre7 features, like... template settings page. So you have to either wait till the panel template will be updated, which I doubt will happen any time soon, update it yourself or use the Default panel template.
Disable debug mode in AdminCP.
That's not really possible since Default is built with Bootstrap which has different themes which that module is using, while DefaultRevamp is built with different css framework that doesn't have such themes. If you want to change the way your template looks you can edit the css and html at custom/templates/DefaultRevamp.
Not 100% sure what's causing the problem, but after changing the domains have you also changed the hostname in core/config.php to your new domain?