What exactly will Russia do if we "annex" parts of Syria?
That's quite a hypothetical question. I have no idea although I would presume that the reaction would be quite strong, involving UNSC action on the similar line taken against Iraq from August 1990.
Check out a string of UNSC resolutions, starting from Resolution 660 and ending up with Resolution 687 in April 1991.
So, you want a Kurdistan in Syria, because this is what American, Russians or Assad want?
I am personally for the territorial unity of Syria under, initially, the present Syrian government. Then, they can move on through internal reconciliation to set up a government that best suits them.
No pseudo Kurdish state should be allowed in neither Iraq, Iran, Syria or Turkey.
Nonetheless, when Turkey got involved in the early years of the Syrian Conflict under your previous PM, there was no such likelihood of a Kurdistan being formed in Syria.
Russia now wants much closer coop with Turkey, why is this? Maybe because they want a real power as an ally in the region?
Russians appear to be ready to communicate any rational actors, including Turkey. They are coordinating war efforts with the US even though they are at each others' throat in other theaters.
Russia is trying to macromanage the situation. It is not possible all the time to get the best result when there are so many actors. Turkey is definitely one of the actors deeply involved in the Syrian War from very early on. It cannot be ignored. But it is not the only actor. Nor is it the most important. The apparent disunity in Turkish response and various disconnected and conflicting voices coming from the government in power and the President also create confusion abroad.
No wonder that foreign policy is always an extension of domestic policy. A good domestic policy translates into a good foreign policy and vice versa.
Russia got legitimacy under international law for all of its actions the moment it received official invitation from the government of Syria.