you might try getting them via the blue arrow + ctrl

this should bring up a few special characters on screen, some of which are diacritical marks only (like ^ or ¨ or °). You can tap on them and then press the letter on the keyboard, so ¨ and a will get you ä.

The only thing I don't know is if all the diacritical marks you need are available. There's an application somewhere, though, to customize this pop-up keyboard.