That's a few questions.
1. How to open HTML with Word.
Goto File, Open and maneuver to the folder with the HTML and open it. You may have to change that pop down to *.* (All files).
2. I strongly suggest you not change HTML to open with Word by double clicking on the file in Explorer.
3. When I use File, or File Save As there is a pop down to select the file type. You don't seem to be aware of that. Here's what Microsoft wrote on that.
"Save as a different format
Click the Microsoft Office Button Button image, and then click Save As.
Important If you don't see the Microsoft Office Button Button image, click Save As on the File menu.
Keyboard shortcut To open the Save As dialog box, press ALT, F, A.
In the File name box, enter a new name for the file.
In the Save as type list, click the file format that you want to save the file in. For example, click Rich Text Format (.rtf), Web Page (.htm or .html), or Comma Delimited (.csv).