Well that was tedious, esp. on my mobile keyboard
why is eenentwintig and not éénentwintig?
Good question. One/één gets the accents to differentiate between a/an/een and one/één. Since there is no such issue with eenentwintig the accents are not needed.
No numerals allowed this way!
I wrote without the two dots on the e's and it was accepted, should it be or is it a mistake?
It's not correct to leave out the diaeresis (trema in Dutch). But in other languages I know this kind of mistake was accepted with a "you made a mistake" warning, I think this should be the case here as well.
I think they accepted it because not many people have the trema on their keyboards, and it's tedious to always click it.
It's probably easier to sort out your keyboard than to continue to have this problem. Especially if you intend to use Dutch outside Duolingo.