My need is to have Site A (regional site) to share content with Sites B, C & D (local sites) with Sites B, C & D to share 'some' content with Site A (regional site). These sites will probably be on different accounts as well as a few might be on different hosting services.

I read this in the FAQs:

Does JMS use the same Joomla Installation

YES, all websites share the same Joomla Installation.

So Joomla Multi Sites cannot share content between different hosting accounts either on the same server or on a different server. Is this correct?

This is correct and may be incorrect.

This is correct that JMS will NOT be able to share or work properly when using different hosting account if you don't have the privileges to SHARE the same files and folders.

The key element is to have a shared disk or at least the privileges to create the bridge between the different servers.

All depend of the security of the hosting provider.

You have have totally different hosting provider like (godaddy for some website, and hostgator for other website) in this case it will be impossible to share the same disk (files and folders).

This is incorect if you are using your own server and that you create the hosting account for your customers. In this case, as you have all the permission to grant YOUR hosting account to share the files and folders then in this case only, this is possible.

The same answer is also applicable with the DBs and the extension sharing.
This will require that you have enough MySQL login (user) privileges to allow for example sharing the users.
Concerning the "Article Sharing for JMS" extension, it is NOT required to grant the user to be able create the DBs with sharing table.
The only constrain will be to create the slave site them self to have the DB connection parameters to all the "Article Sharing" connect on the appropriate DB.

In summary, in most of the case, this is correct. JMS will NOT be able share the Joomla PHP code between the hosting account.
The only exception concern the case where you are your own hosting provider and that you can grant the hosting account with the privileges to share the files and folders.