How do you pronounce Margreet? Margreet or margreat (Dutch prn.)? If you prefer the English prn you could also use Margriet which is prn mar-greet. The nn Griet is easier to use as a given name with a middle name, re your concerns with Greet. Plus, a Margriet is a flower :)
Other Dutch names you might like: Marlies, Marloes, Annelies, Annemiek, Marieke or Lisette?
Since we live in and English-speaking country, we tend to Anglicize pronunciations for simplicity's sake, so closer to Mar-greet than mar-great. I also have always loved the form Marguerite, which is nearly identical to Mar-greet. I'm open to Margriet, but dh's name is Niel, and his name is constantly spelled Neil. And I can't tell you how many times our son has come home with Lieden on his papers. So he doesn't want to give people the opportunity to have ie ei confusion this time around (which is why he doesn't like Evelien as much as I do. I love Lisette and Lisanne/Lysanne, but dh was not as keen. He doesn't want to repeat any initials.