"I see these daughters."
Translation:Ich sehe diese Töchter.
You're half right. It is accusative, but "diese" is a definitive article, just like der/die/das. See this link:
Has nothing to do with definite article. "Töchter" is plural. That's what determines to use "diese". If it was "Schweine" or "Männer", although they are neuter and masculine in singular, it doesn't matter when they are used in plural.
Ich sehe diese Schweine.
Ich sehe diese Männer.
Ich sehe diese Töchter.
All three genders used. Still, they go with "diese", because they are all plurals.
Every umlaut can be replaced by adding an "e" to its base letter. E.g. ü is equivalent to ue. Duolingo should accept that. I don't recall if I tried, so can't confirm that they do, but they should.
In fact, in German crossword puzzles that's the only way umlauts are accepted.
But, at least Duolingo's web interface provides means to enter umlauts and the ß, so you should not have any problems entering the proper letters. There are buttons in the web interface for these.
If you are on an Android device, hold down on the base letter of the on-screen keyboard until a menu of special characters pops up. Select the umlaut and you're set.
If you are on a Windows PC, you can also use the following keyboard shortcuts:
That is: press and hold down the Alt-key on the keyboard, then type the three-digit code on the number keypad, release the Alt-key and the umlaut or ß appears.
There is one catch though: the letters whose codes contain a 4 will not work directly in a browser, since Alt-Num 4 happens to be the keyboard shortcut for "the "Page back" function in most, if not all browsers. So, you will lose your place if you try to enter ö, Ä or Ü in a browser form. A workaround is to use the key combination in a different application, for instance, Notepad and then copy and paste the umlaut into your browser.