You can see a little more info + a “map” of how they interact here:
From what I understand, the versions of Nextcloud/richdocuments/Collabora can be interchangeable… because they each cover different “layers”.
For example:
Nextcloud- = A server where users can login / save+move files around.
Collabora Online- = The actual document creation/loading/editing.
richdocuments- = How Nextcloud integrates+communicates with Collabora.
- For example:
- Syncing usernames+cursor locations.
- Sharing Nextcloud’s file permissions.
- Some people can read-only. Some can fully edit. Some can’t open at all.
- Easily “Share” URLs to your Nextcloud document.
So you can upgrade Nextcloud, or upgrade Collabora, and as long as they are still speaking the same API/language, they can still communicate with each other.
(Of course, newer versions will probably fix many bugs and be better than old! :P)
lol. Well yeah, you’d get a painless and working install. 
I’m unsure.
Were you able to get 1 working correctly?
Did you happen to see the relevant section in:
or the:
[…] When two users join to the same document but doesn’t see each other edits or saved content is lost or overwritten by other users working on the same file.
That means the HTTP requests related to the same file are not directed to the same Collabora Online node.
Read more at PostMessage API.
Hope that helps. 