How do I create Global Categories & various others.

I have been searching high and low for an answer to these questions.

How do I make the Main Site as the place to write posts and is distributed to other sites on the network depending on the Categories/Sub Categories selected?

How do I show NextGen 2.0 Galleries across multiple sites on the network from the One Main Site hosting it?

Here are some examples for the first question if it is a bit hard to understand.

I have made a Network for locality reasons only (Language, Different Pricing, Different Events).

1. The Main Site is example.com
2. The AU Site is example.com/au/ with a domain mapped to it as example.com.au
3. The UK Site is example.com/uk/ with a domain mapped to it as example.co.uk
4. The NZ site is example.com/nz/ with a domain mapped to it as example.co.nz

The Main Site will have ALL posts/content/galleries. The category structure would look like this:

Galleries: AU | UK | NZ

When we crate a post and tick only the AU Sub Gallery Category, we want only the AU site to show that post and of course also show on the Main Site.

The posts will include the NextGen Galleries which will be shared across the Sites where we have ticked the category.

Any solutions like this, plugins, hacks. Or should we not use Multisite?

Thank You