Drupal 8.x

I have just started looking at Drupal 8 having got part the way through the development of a module under 7. Im trying to put a tab on the main navigation menu alongside the 'home' tab. Under 7 this was achieved with the following from my original project.

Have just setup a new Drupal 8 site and I have 3 custom content types:

-Company
-Contact
-Ticket

Each contact is linked to one Company (Contact has en entity reference field for Company).

The ticket content type has 2 entity reference fields, one for company and one for contact

When I create a new ticket, I first select a company, then I want the list of contacts to be updated to show only contacts linked to the company selected above. I'm sure this is easy to do I just can't figure it out!! Can anyone help?

I have a custom module with an Interface. No matter what I do I keep getting Interface not found in php file (the one that has the use statement and implements it).
I've even gone so far as to put it into a core module directory that I'm referencing other Interfaces from. Those
references are fine but I still get Interface not found on my Interface.
Any help would be reatly appreciated.

I have an active D7 site with a dozen + content types and hundreds of nodes, many cross referenced to each other. If I perform an initial migration, that will give me a baseline content dump to start re-configuring and re-theming from. However I am expecting that it will take quite some time to get all the pieces working together properly. During this period the production site will have had many updates.
Is there any workflow/technique that would allow for a secondary migration/overwrite/dump/merge-purge that could deal with the changes between the old/new sites?

I've been testing the views module for some days with REST enabled. I want to create a GET request from external websites. Now I'm confronted with the cors issue.
So do I have to use the rest module because of this issue?

I am installing drupal 8 for the first time, I have gone thru all the pre-installation issues, and have changed the permissions on the subject folders several different ways going as far as allowing 777 temporarily and yet the File System issue will not allow me to go beyond the Requirements page during install. What else am I missing?

Thanks,
Tony