At first glance, there are a few enharmonics (same note represented differently) to fix:
The D sharp in the soprano 1 in measure 5 should be written as an Eb.
The A sharp and B sharp in the soprano 2 in measure 7 should be written as Bb and C, respectively.

I quite like this. The last three measures slowly introduce a level of dissonance, which resolves quite nicely to the open fifths at the end. If you find that tuning is difficult at the end, you could simply nix the top soprano 1 split and change the last note of the soprano 2 line to a G. This would accomplish a similar, if slightly less "spicy," effect that would be much easier to tune.

I assume that it’s because of the chant included— but of course one can export a document made in a word processor to PDF or even manipulate the PDF to include extra elements.

As an aside: while I do mostly like Musescore, I would encourage people interested in including sqaure notation alongside polyphonic music to consider Lilypond; there is a LaTeX package which would allow you to insert polyphony in the same document as a Gregorio score.

No worries. I have had some success with Musescore, but it’s not perfect by any means.