'ku' taken from the Russian word 'k'
Nope. Wrong again. You should really put away that soviet linguistic gibberish that you're soaking in.
The preposition is very ancient and predates any mutual exchanges which took place after the languages differentiated sufficiently to be considered distinct.
The word "ku" appears in Wendish/Sorbian dialects, as well as in Illirian languages such as Albanian. Did Illirian peoples wait a few centuries for the Russian to cristalize and then to incorporate the word into their languages?
In Russian "ku" looses the vowel (in some cases) and it is a simplification of the PreRussian "ku".
Just thought you might wanna know.