1) Concerning the creation of a site map extension that read the articles of each wesbite and merge the result to give you a global site map, this is effectivelly something that does not exists.
I don't understand the reason of this requirement as you can have as many site map as you have websites. Such individual site map can be generated with XMap or other extensions.
2) In the "Tool" menu You can unsinstall extension and propagate to the children like you can do it with the install. You just have to create group of websites to have children.
3) Partial sharing is something available with the "Article Sharing for JMS" extension.
So you could have a publisher website (ie your master) where you put the article that are common and create in the slave the menu to display the shared articles.
www.jms2win.com/download?page=shop.produ...34&category_id=1
See also the tutorial video 0.a slide 11 and 0.b slide 41-44
www.jms2win.com/tutorial#jms12x-0a
www.jms2win.com/tutorial#jms12x-0b
4) Create an article with field area that would be "common or shared" and other speciifc is something that you can do with the Search/Replace.
We don't see the interest to create another plugin to do the same.
With Search/Replace, you can also use files that will be computed with share data or specific data. This is not a hack but a system like any mailing tools that replace some keywords by their real value.