Hi Paul,
In response to your questions:
Is data content physically replicated across the different portals, or merely items kept in sync that allow a user of one portal to see items from other portals and access them?
Yes, most of the data will be physically replicated between the different portals. This applies to almost anything that can be stored as an item in Portal under My Content: web maps, web apps, shapefiles, map packages, tile packages, service definition files, etc. The exception is map service and feature service data. In 10.5 the service items will be copied over by reference. This means if you have a map image layer or feature layer in one Portal, the item will be copied to the collaborating Portal but it will still access the data through the original url. There are plans to replicate feature service data between collaborating Portals in 10.5.1.
Is it possible to connect a Portal instance to an AGOL account?
No, for 10.5 a Portal instance cannot collaborate with an AGOL account. That is another feature we plan on including in 10.5.1. When this is supported it will include the database replication between Portal and AGOL as well.
