WP Multisite + GF User Registration Add-on + Post to blog question

1) Gravity Forms does indeed only create posts and users on the site it's submitted from. It could do user creation and site creation on a form, but it does not currently do theme selection or create posts on the newly created site. If you used Post Fields on that same form it would create the Post on the site it is submitted from and not the site that the User Registration Add-On has created when creating the new user.

What you want to do is more complex and more of a custom use case. You couldn't certainly do it with Gravity Forms but it would require custom code and using available API hooks and filters to customize Gravity Forms functionality to suit your needs. As for how to build it, it's not a simple question and would require fully scoping out what you want to do, etc. and since the exact workflow you want isn't how Gravity Forms currently works that becomes a customization at which point it's something you a developer would need to implement.

2) If you needed posts to be created via a form you could certainly configure things so that when a site is created, some pre-defined forms are also created. However, it's a catch-22... since no pages or posts would exist with that form embedded there would be nothing to link to. So as part of the customization you would have to implement to accomplish #1 above you'd also have to script creating a page on that new site that contains the shortlink to display the form you automatically generate in #2.

You could do what I describe above without requiring them to have access to the GF admin. But you'd have to leverage WordPress role management capabilities to do so if you are granting the new user Admin access to that site. If the user doesn't have admin access, he wouldn't see the GF admin area anyway.

What you want to do is a very custom use case so it's going to require customizations if you are comfortable making them. Gravity Forms has an extensive library of hooks and filters and is extremely customizable. Just like themes and plugins customize WordPress, they can customize Gravity Forms in the same way through these hooks. BUT it does mean you need a developer who has a good grasp of WordPress development and understands hook/filter usage, etc.