If you open this epub in the calibre Viewer, does the cover now stretch edge to edge, even when you drag the bottom left corner of the Viewer to vary the size and shape of the Viewer window? Based on the code you posted, the answer should definitely be 'yes'.
I think the next thing I'd do is to delete that particular book from the Nexus, using Calibre Companion. Then exit from CC and go into Moon+ and make sure Moon+ also thinks the book is completely gone. Then go back to CC and send the book again and hope that Moon+ sees it as a brand new book and isn't in some way 'remembering' the old book in some sort of cache. I'm also not sure whether Moon+ is one of those aggravating apps which insists on making its own copy of the book in its private storage, before it will open it.
If you still have no luck with that book, temporarily import it into your calibre library as a new book with a slightly different Title (and a new calibre book id number). Try sending this new named book to the Nexus from CC. Are the results any different? Don't forget what I said yesterday, Moon+ doesn't seem to be able to process SVG covers completely correctly, as it won't do the stretch/distort to ensure edge to edge. But on my phone the 'filler' is grey bars not the text content from the next page.