< in messages

You give a proof with a specific page used only by admins. But there's no double encoding of ampersands in Mediawiki. Everywhere else the "<" have to be escaped to avoid incorrect matches as candidate tags.
valid numeric entities and at leastr the few basic standard characters (which are not "named entities" in HTML, but part of its required syntax) are implicitly recognized (they do not require any DTD, their supprot is mandatory in all HTML and XMS parsers, and in MediaWiki itself): "lt", "gt" for tags, and quotation marks and required for attributes and ampersands for URLs; the support is extended to numeric entities since HTML4 and in all versions of XML and mandatory in HTML5. It is mandatory since ever in all versions of MediaWiki (initially however it had limited support for Unicode and allowed numeric entities mapped to 8-bit only codepoints before UTF-8 became mandatory, which occured when HTML was fixed to require Unicode-only mapping of numeric entities, forbidding mappings to legacy codepages, even if the pages themselves would still be encoded with legacy charsets, this no longer applied to numeric entities).
You're living in the early non-standard age of HTML of the end of the 1980's when MediaWiki did not exist, Translatewiki.net did not exist and HTML was full of ioncompatible proprietary versions derived inconsistantly from the initial CERN invention.