Hello Charles
Thank you for replying! The problem that some of my clients have is that it doesn't just restrict the Home tab. It also means the Breaks button in the Layout tab is disabled; it also switches off several commands in the Table Design and Layout tabs. For some financial sector clients, that's a big no-no.
That's why I wanted to change the XML file and amend the tabs and commands instead. Some clients want certain commands like Bold, Italic and Underline to work, so Restrict Editing is out-of-the-question. Changing the XML is perfect in that I can switch off the Font dropdown menu and keep the buttons they do want; I just need to switch off the Modify Style and New Style dialog boxes, but I can't find a way - and I've been looking for years now!
I know of no XML or vba method to do what your clients want. That may be speaking to my ignorance rather than what can be done.
Using XML Ribbon modification, you can certainly remove the font drop-down, but not the right-click menu unless you also modify that, which can be done. Customize Text Shortcut Menu by Greg Maxey
The restrict styles, though, can still allow use of bold, italics, and underlining through character styles. The sample I prepared has styles for both Bold and Italics (Strong and Emphasis) as well as combinations. They are used in the sample. The same could be done for numbering (attach to permitted styles).
Other than styles restriction, I know of nothing to cure this minitoolbar:

With styles restricted, you get most formatting commands including the font dropdown disabled as shown:

Apparently the restriction of styles gives the Reviewing commands on this minitoolbar even though Track Changes has never been implemented and there are no comments.
There is no vba or XML available to change this toolbar.
With Styles Restriction applied, this is the Styles Pane in the Sample Document. This is with the options set to display all styles in alphabetical order. All of the styles shown in the Styles Pane specifically had to be permitted except Clear All, Default Paragraph Font, and Normal.

Only those that are in the [Quick] Access Toolbar show up using the Styles button in the minitoolbar.
You are correct that substantial portions of the Table Design and Layout Tabs are disabled.

The Table Design issue might be addressed using Quick Tables. The things disabled in the Layout Tab could be addressed with a Custom Ribbon and vba commands that would emulate the commands otherwise available. Certainly the Breaks issue could be addressed with vba. I expect Columns could as well. Customize the Office Ribbon (It doesn't take rocket science) by Greg Maxey
I think style restriction through Manage Styles is as close to what your clients want as you can get in Word.
You can ask Microsoft for even more control, but do not expect a response other than a possible product change.
Giving Feedback on Microsoft Word or other Microsoft Products