Unfortunately, I was hasty and often incomplete.
A red flag with a cross is not the Serbian poetry. Serbian folk songs have been devised by the beginning of the nineteenth century. How could they know the Montenegrins and songs for centuries earlier? The cross is a religious symbol and flag of Montenegro has the function to mark the Christian army that is fighting against the Muslim Empire

Nope, but it was few different flags, and without any codification.
Do you have it right.

Yes, I think I emntioned Metropolitan banner. But it was banner of Bishopric not of state. Hope you see difference.
Do not flag Metropolitan was the emblem Crnojevića (in red gold double eagle). a flag with a cross was the flag of the Montenegrin army troops.

It was not something Turkish sultan awarded. Serbs in Austria designed that flag, and used it from end of XVIII century. Than in 1835, Serbia voted first Constitution. Tricolour was proclaimed Serbian flag. In 1838 Austrians and Russians, who were against liberal Serbian Constitution forced Sultan to issue new constitution for Serbia, which kept tricolour. I guess no need to go in details, why it was not Sultan who granted it.
The first flag of Serbia was a red-white-blue with the coat of arms of Serbia (the red cross with a silver-steels). I think the story of opposition Austruije and Russia’s funny, because Sultan introduced again tricolor flags with those that are blue and white switched places. True Austro-Hungarian Serbs have previously used this flag.