Is that your rationale?
The constitution is not the be-all and end all. It's just a piece of paper, and without support of the people of the country it is a worthless piece of paper. It does not supersede the wish of the majority of the people. Ask the USA when they split from the British Empire if the law of Great Britain at that time allowed for secession. And as said Kosovo separation resulted in a war where NATO killed over 1,000 civilians. So seems like a double standard to me.
You haven't made any valid points. If the referendum is free and fair, the area should be allowed to separate. Next you'll tell me that persecution of Jews in Nazi Germany was legal because their constitution or law said so. Nuremberg trials disagreed.
How can the referendum be free and fair when...
1. International monitors have been disallowed from observing to make sure its free and fair.
2. They are occupied by a foreign military that has an interest in seeing the referendum go a particular way.
3. The referendum leaves no option to remain a part of the Ukraine, it's basically leave Ukraine and join Russia or just leave Ukraine what about all the people who want to remain a part of the Ukraine.
4. This region is not a representation of everyone, if you went to a place like Oranje?? I am sure a referendum to break away from South Africa may succeed but that doesn't mean it should be allowed.