ok, like I said:
As was confirmed by Moyogo (thanks): avoid using (substituting) spacing accents.
Instead of dreaming about OS makers fixing all their keyboard layouts in the same fashion in the near future, lets collect some practical input methods that work today and educate users…
1
A Guaraní or Bulgarian Windows user can memorize the Unicode for their special accent, like 0303 for combining tilde, and 0300 for combining grave. Type these in MS Word, then Alt+x. This turns Unicode into glyph, and also works the other way round, very practical.
2
Open the Windows character map application, click Advanced view and search for “combining”. From there you can insert any combining accent (there are many) into any text, regardless of application. Combining accents might not be visible until you type a letter or a space in front of them, so it works best if you first type the base character in the empty box, then double-click one or more combining accents, and copy the combined result.
3
On Mac it needs some preparation. System Preferences > Keyboard > [v] Show keyboard and emoji viewers in menu bar must be activated. Then choose Show Emoji and Symbols from the right side of the menu bar.
This does not always work. Try again until you see a window. The position, size and visibility of that window is coupled to the application that happens to be active, so it might look confusingly different at times. The window that appears might only show emojis, in which case you need to click on the right top button so you get to see columns, it is named Character Viewer now. Don’t give up. Even though no scaling cursor is shown, this window can be scaled bigger if you grab it accurately by the edges. (Apple UI, grmbl)
Now type “combining” in the search box. It will find the characters: c o m b i n i n g for you, very smart, but below that, under Unicode Name, it will find endless combining glyphs, even many that have no image. Select the combining accents you need, and in the rightmost column, click on “Add to Favourites”. Now they are easily available, double-click inserts them into text, even when the panel reverts to its smiley-state. (This panel changes with every OS version, and was always troublesome, does it still mess up things when shown on a 2nd screen that gets disconnected?)
4
On an iPhone I have no idea how to type combining accents, but if you choose a Bulgarian keyboard, press vowels shortly and not too firm, for some of them a variant with grave will show already. Someone from Bulgaria might need to give Tim a hint, no wait, Apple doesn’t process hints 
5
You can use third party software to find and insert combining accents, like PopChar, which already lived on my oldest macs, also available for Windows, or Char Menu Light, from type designer Michel Bujardet, in the App Store.
6
what else?
Whatever the input method, the type designer decides if the result is a pre-composed thing (requires ccmp), convenient for InDesign, or if it is built with the anchors mechanism, convenient for arbitrary combinations.