Let's expand the second sentence:
Vorerst muss ich noch über den Sonnenbrunnen wachen oder vorerst muss ich noch über das wachen, was davon übrig ist.
Where was refers to das and davon refers to Sonnenbrunnen.
Let's shorten it further:
Vorerst muss ich noch über den Sonnenbrunnen wachen oder _____ über das ___, was davon übrig ist.
Vorerst muss ich noch über den Sonnenbrunnen wachen oder _____ das ___, was davon übrig ist.
Vorerst muss ich noch über den Sonnenbrunnen wachen oder ____ , was davon übrig ist.
So there are two verb-second main clauses, of which the second is mostly left out, and only the new information is kept : "was davon übrig ist".
This is a relative clause, whose reference in turn was left out, too. When this is done, the relative clause is also called a free relative clause.
In English, I think, there is no comma necessary, but in German, the (free) relative clause needs to be separated from the clause it is embedded in with commas.
I now assumed that this sentence uses an mostly omitted main clause. Someone else may claim that that's not even the case here, and "oder" connects two adverbials, maybe.