I’ve tried space. About sharing: how about having unique textual IDs (say, UUIDs) for each space entry, that get carried around in cards and used to update cards in place?
Then space entries.spc deckname would update the entries in deckname, but also invent and add such IDs to any entries in entries.spc that are missing them. Then all we need to take care with is to only check in spc files that have IDs for all entries, which also serves to ensure the spc file is used at least once before checking in.
Since the ID is per space entry, not per card, need to figure out how to deal with intervals. Inheriting the smallest interval in any related old cards for all new cards seems just-workable, but not elegant at all.
But I think the proposal of this page is great, and update would be necessary, so why not. The other approach that might be better would be to change anki itself to use textual sources natively, but that would probably be a much larger change.
The choice you offer is false, in my opinion. If you violate copyright law, you will never gather a community effort, because who wants to work on something that can get DMCA’ed out of existance at any moment?
I think CC-BY-(maybe SA) will work fine, and just use appropriately licensed basic sources like wikipedia.