It turns out it was indeed wise to apply for the Chinese Visa in Tehran as it couldn’t have been any easier. All it was required was a filled application form, which included the itinerary details, a passport photo, and a visa support letter from the German Embassy in Tehran. Apart from that, no supporting documents, no booking details, no flight tickets were asked for.
Even the photocopies of the passport were returned. I applied for the express visa which costed 70 USD. There was no waiting time at the Embassy on my two visits. It took only five minutes to submit the visa application, and I was asked to come back after two days. Today I went to pick-up the visa, and it lasted not more than 2 minutes to have my passport with this beautiful sticker inside.
The only difficulty I faced was the journey to the Chinese Embassy located in the northern Tehran, a 90 minute journey from where I am currently staying.
From today, I have 89 days to reach Kyrgystan-China border.