Jason Underdown reports on 2016-05-12 that emacs' nXML mode (www.gnu.org/software/emacs/manual/html_mono/nxml-mode.html) works well with a RELAX-NG schema. While we work on building a hand-crafted RELAX-NG schema, you can use the trang (www.thaiopensource.com/relaxng/trang.html) tool to convert the PreTeXt DTD to a RELAX-NG schema.
You simply put your cursor at any point in the document, start a new tag with < and then call the completion-at-point function (I bound it to the key-chord: C-<return>) to get a list of possible completions. Or you can start typing a few characters to narrow the list of possibilities. It will also let you know if the element you are trying to insert is invalid.