The issue of restoration of the Astan monarchy was discussed before, during and after the negotiation of the Bonn Agreement in late 2001. See these links for a summary:

US & Afghan Positions 2002-2008 - part 1

US & Afghan Positions 2002-2008 - part 2

US & Afghan Positions 2002-2008 - part 3

From the second link:

After the fall of Kabul in November 2001, the United Nations invited the major Afghan factions, most prominently the Northern Alliance and that of the former King - but not the Taliban - to a conference in Bonn, Germany. On December 5, 2001, the factions signed the “Bonn Agreement.”
and:

Permanent Constitution.

An “emergency” loya jirga (June 2002) put a popular imprimatur on the transition government. Former King Zahir Shah returned to Afghanistan in April 2002 for the meeting, for which 381 districts of Afghanistan chose 1,550 delegates, of which about 200 were women. At the assembly, the former King and Rabbani withdrew their candidacies and Karzai was selected to remain leader until presidential elections.
So, history has it that the issue was resolved by the Afghanis and by the former King.

Given the political and military environment in Astan, I can't see how any sort of non-speculative answer can be given to Fuchs' question:

Was it a mistake not to recreate Afghanistan as a constitutional monarchy?
but if the answer is "yes", the Germans are obviously to blame - they hosted the Bonn conference.

Have fun with alternative history. I'll pass.

Cheers to all

Mike