I’ve just found that content control <w:std> is more appropriate for templating than MERGEFIELD. So, the problem 1. solved. The <w:std> is easier to find. I am just not sure how to use <w:alias>, <w:tag> and content control text to be convenient for the users who would create the DOCX templates.
And finally found this blog with the solution https://blogs.msdn.microsoft.com/brian_jones/2008/12/08/the-easy-way-to-assemble-multiple-word-documents/