I thought diminutives took "het" and thus don't take an e on adjectives, is plural (hence "de") more important?
All singular diminutives use het, all plurals use de.
"Het" words in the singular also take the inflected form of the adjective when the definite article is in the sentence:
"zij zijn te wachten"
Is this an acceptable translation
No, but you can say 'zij zijn aan het wachten'.
No, I used this construction and I was marked wrong. Using "aan het" here didn't work for me. "Zij zijn op belangrijke telefoontjes aan het wachten."--Wrong!
I think it's should be accepted.
This is correct dutch, you can make it shorter but there is nothing wrong with it.
It seems strange to use a diminutive with "important". Does this sound normal in Dutch?