Using MuseScore, I wish to re-typeset a PDF scan that was not originally submitted by me, but I have a couple of questions:
What, if any, changes am I allowed to make? For example, a lot of the hairpins are enclosed in parentheses, some fingerings just don't work, or there are actual printing/typesetting errors. Can I remove or edit those? I've kept the page and measure layout the same and added measure numbers.
If I do make the above changes, should I update the existing score, or should I submit a new file entirely? I'd hate to clutter up the page with something that's really similar to something else.
Since I'm here, is it as simple as submitting the .mscz file? Or a both that and a PDF? Is there a generally used convention or best practice?
As the Joachim edition published by Simrock in 1905 is in the public domain you can make any changes you desire to the edition you are creating that is based upon it.
You would not upload your typeset as a replacement for the 1905 Simrock edition. Your typeset would be uploaded under one of the Creative Commons licenses with you identified as the editor. You would also probably note that it is based upon that 1905 edition and it would most likely be placed beneath it on the page.
A PDF file is required. If you also wish to upload the mscz file you will need to add it to a Zip archive and upload the archive file. It can be uploaded at the same time as the PDF as it should reside alongside it.