Can a Captivate 8 cp file be updated by someone using Captivate 7?
(if we don't use the specific functionality which is unique to Captivate 8 but just the items common to both?)
Don't try it, probably you will not even be able to open a CP8 file in CP7. Look at the Object style manager: there are a lot of changes between those two versions, even for non-responsive projects.
In Captivate 8, can we do a "save as" somehow and select Captivate 7 as the version, like we can do with Microsoft Word? (In Microsoft Word we can save back to an earlier version if we need to).
Nope, sorry. It's Adobe, not Microsoft.