Well, I have to admit that I don’t know that, but linguistically spoken it might be possible: [ć] and [k] are plosives or stop consonants, just like [t], so it seemst to me that ćek and Proto-Slavic *teťi are related. But I have to admit too that I only studied Germanic linguistics (High Germand and Dutch), not Slavic linguistics. I’m very interested in grammar, but at this point of my acquisition of Russian and Lower Sorbian I’m paying more attention on communication.
Is „ćek” unknown in the other Western Slavic languages Polish, Czech and Slovak? I read that the two Sorbian languages did preserve some language phenomenons which are not longer used in the other Slavic languages, like for example the dual, the number between singular and plural.
I do not think that the Rs are supposed to sound like that. Of course, modern day speakers of the Sorbian and Wendish languages in Lusatia are influenced by the High German language. This is especially true for the urban population. There are probably also Sorbian families who identify themselves as Sorbs, but in which the knowledge of the Sorbian language might no longer be so good. So they are ethnic Sorbs, but German native speakers (as well as that is the case with many Slavs in the US?). Although, they learn the Sorbian language, but often keep the German basis for articulation. In the more rural areas, the Sorbs and Wends are said to have a more typical Slavic articulation.
Native speakers of High German, at least in Northern Germany, actually realize only a very weak, vocalized R. So words like „Mutter” (mother) or „Bruder” (brother) sound almost like „Mutta” or „Bruda” … but not quite like an real a neither. Language is something interesting! I think it has to do with the first-syllable accentuation in High German. Maybe in some centuries in German we’ll speak also from Mutt’ and Brud’, like Russian Мать and брат.