This photo at the Daily Mail resolves the mystery. It clearly shows the flowers were a gift from Turkey:
Caption: Turkish honour guards stand after they placed the coffin of Russian pilot Lt. Col. Oleg Peshkov inside a Russian Air Force transport plane at Esenboga Airport
The Daily Mail also published this photograph, taken in Turkey, as the hearse was leaving for the airport:
Caption: "A final guard of honour is given to the dead pilot as he leaves for Russia"
By the way, thanks to the Daily Mail for providing Peshkov's rank. The reports I saw at Sputnik and RT (Russian press outlets) hadn't done so.
All right. Now what can we make of the flowers? From all the photographs I've studied, each phase of the transfer of the body was handled by Turkey with the respect they would accord a fallen Turkish war hero.
At the least the flowers convey that the Turkish government still considers Russia an ally. I'd venture they also indicate what President Erdogan would not say in words -- that he regretted the pilot's death.
What's the old advice? Say it with flowers. So. A floral arrangement may have averted World War III.