Since the use of different versions of a recording are so common, like remixes or acoustic versions, why isn't there an option to specify that in the relationships? For example "remix recording of", "acoustic recording of", etc.
There are recording–recording relationships for remixes.
What do you mean by acoustic recording?
This would already be stored in MB, in the sense that the instruments used should already be present in the recording-rels (as should the work being performed).
But also consider using tags where appropriate
So should I just use the work that's associated to the original version of the recording for the remixes as well?
Absolutely. And if it is performed by another artist, check the cover attribute.If a part is missing (incomplete lyrics, removed verses, etc.), check the partial attribute.
Depends on the extent of the remix. If it's basically the same lyrics and basically the same melody (maybe the rhythm got shifted a bit for a dancehall or jungle or hip hop or tropical or Mike's Amazing Party feel), use the same Work. If the remix does something like replacing all the verses and just keeps the chorus or sets the original song/lyrics to an entirely new tune/melody, then make a new Work (and relate the new Work to the original one).