Letterhead
The standard method in Word for letterhead is to place the elements in the headers/footers.
- [http://wordfaqs.ssbarnhill.com/LetterheadRibbon.htm]
- https://addbalance.com/word/headersfooters.htm#2007-2013
If you follow that practice, which I recommend, items in the headers/footers are not visible in Word Online when in Edit mode. They will be visible in the desktop versions of Word and will print.
Cover Pages
I recommend saving a Cover Page as a Cover Page, not as letterhead. Then it will be available in all versions of Word. You can create such a cover page in the desktop versions of Word by inserting a Cover Page in a Word template and then replacing its content with what you have designed. I assume that what you have is a graphic.
Then you can select your page, less the manual page break, and store it as a Cover Page Building Block in the the gallery. You can make yours appear at the top of the list by giving it a name that starts with an underscore.
Cover Pages in word act a bit strange in that they are not counted in the page numbering, at least in recent versions of Word. See Cover Page Anomalies. That link gives information on how to store your cover page in the gallery, as well.
If you want your user to be able to type in it, position the Graphic to be behind text. If you want certain things to be on your cover page, I recommend exploring the ones that come built into Word and looking at the elements in them that allow typing.
Here are a couple of samples built into Word. They have spots for the user to type. The information typed there can be automatically and dynamically repeated in other parts of the document. Repeating data using document property content controls.
If you really want to do a good job for your client, save your Cover Page as a Cover Page Building block in a separate template to be placed in the user's Startup Folder. That way, they will be able to insert it in any document using the Insert Cover Page method. This is not cloud storage.
The same thing could be done with letterhead saving the letterhead as a Header building block and telling the client to use it with a Different First Page header. (Or, you can save your letterhead as a Word template itself and give it additional features such as a custom continuation letterhead as described in the second link given above.)
Where can Building Blocks be stored? shows how to store building blocks in different templates.
These work in Word for Windows desktop application. They do not work in Word Online, which is a separate program. As far as I know, building blocks do not work in the Word Online program. They would have to be modified to be used in the Macintosh desktop application. Custom AutoText Galleries on the Mac
I store my custom building blocks in files which are in cloud storage, but they are synced to desktop files.