GRAMMAR
Personally, I don't see a real problem. A person using the site (origin of messages) will know how to use these templates correctly. If there is an error, the translator will come back here to correct it... And if he's nice :) he will complete the documentation ("Information about message") to help others so that he doesn't the same error!
Yes but a template-based solution will not work on all wikis if these are generic Mediawiki messages (e.g. to be deployed to Miraheze, or elsewhere). The doc may however be linked and shared on Mediawiki-Wiki. Such thuing cannot be decided isolately.
It's already decided. There is a Grammar parser function and we want to utilize it only for SITENAME and Usernames. Why are you complicating this even more? "message sent (sender: $1; recipient(s): $1)" is not always possible to implement and we need declensions for SITENAME. For example we have to omit SITENAME on MediaWiki:Searchsuggest-search/ku-latn or use "SITENAME: Lê bigere" which is not ideal at all. We encounter SITENAME in translations enough to require utilizing something that already exists. Can we stop discussing issues in Localization please and start doing something about our problem?