It appears that not all of this is possible with the current version of the DocX library you are using. If it is not exposed in documentation, the functions might as well not exist, and you should not be using undocumented features.
There is a much more mature library available, however, called the "Open XML SDK", that can do everything you need.
The correct way to change a font, regardless of whether you are doing it with the document editor, or you are writing a program to manipulate these files, is to change the appropriate text's style attribute, or changing the definition of style in use.
You should never, ever, ever, ever directly change the font of any text. Personally, I think that the 'font type' and 'font size' menus should be removed entirely from word/libreoffice/etc, and only be accessible inside a 'change style properties' dialog; the only reason to directly apply a font is if you are actually providing an example of particular typeface under discussion!
See How to: Replace the styles parts in a word processing document (Open XML SDK) from the MSDN documentation for a description of the way that works.
To search and replace text, the applicable MSDN page is How to: Search and replace text in a document part (Open XML SDK). For specifically replacing multiple spaces with a single space, there are numerous results on Google that should all work to at least some degree.