xpath-bugfix.patch affected only xml values containing an xml declaration with "encoding" attribute. In UTF8 databases, this latest proposal (xpath-parsing-error-fix.patch) is equivalent to xpath-bugfix.patch. In non-UTF8 databases, xpath-parsing-error-fix.patch affects all xml values containing non-ASCII data. In a LATIN1 database, the following works today but breaks under your latest proposal:
It's acceptable to break that, since the documentation explicitly disclaims support for it. xpath-bugfix.patch breaks different use cases, which are likewise acceptable to break. See my 2017-08-08 review for details.
The fact so this code is working shows so a universe is pretty dangerous place :)