A family of Microsoft word processing software products for creating web, email, and print documents.
There are some keyboard shortcuts that are simply not available.
I know of no list or documentation on this.
See my note about reserved key combinations. Assigning Keyboard Shortcuts in Microsoft Word 2007-2019 (365) Two that I ran into include the F1 key and for whatever reason, Ctrl+Shift+Alt+D! My system shows Alt+R as unassigned, even though it will trigger the Reviewing Tab.
I generally avoid assigning anything to simple Alt key shortcuts because of the ribbon shortcuts that pop up with Alt.
Consider using a trigger or prefix in your shortcuts so as to make them more memorable and so you have more available. That is discussed in my article.
Ctrl+Alt+2 is the default shortcut for Heading 2. While I realize you are used to your custom styles, there are real advantages to the built-in heading styles. You may want to simply modify their appearance and add alias names to them. Why Use Word's Built-In Heading Styles? by Shauna Kelly
Also, keep in mind that you can assign your own order to styles, including the built-in ones by changing their priorities. The Manage Styles Dialog by Suzanne Barnhill, MVP Those settings change the order of appearance of any in the Quick Styles Gallery and will change the order in the Styles Pane when you choose "as recommended" for the order. (I always use alphabetical, myself, though.)
Long term, consider saving your shortcuts in a template (other than the Normal template) that you use as a *Global Template*. This makes them much more portable and means that if something goes wrong with the Normal template, you do not lose them. You can use *Chris Woodman's Keyboard Shortcut Organizer* to copy shortcuts in Windows systems.
I admire you and your wife for your use of styles. It puts you in the ranks of those who are really using Word rather than let it push them around.