Indeed in the Proto-Indo-European language there is a known phenomenon called "s-mobile". Some roots in PIE could occasionally have an initial augment "s-". Since all PIE roots begin with a consonant, this gives initial "sC-" cluster. The meaning of this extension is not known. We only know that some words with a root had it while others did not.
There is a theory that this initial s- could be result of fusion between the usual PIE ending -s/-os with the following word.
But most of the examples you gave are irrelevant.
The word for star in PIE was a̯ster, with the first phoneme being a laryngeal which was either lost or became a vowel. It is conjectured that the initial form was even more complicated: a̯e̯ster and related to a̯ee̯tr, "fire".
Russian "strana" on the other hand, sometimes considered to derive from a root with s-mobile "(s)tero̯-" (Pokorny) which meant "stretch" but this root may be considered s-mobile erroneously due mixing with other roots (Starling gives this root with a solid s).
The origin of the "schnell" and "snell" is unknown beyond Proto-Germanic.
The word "stamina" origins from the root "stea̯-" which had no mobile "s" at least in the observable PIE stage.
The origin of the "spyros" is also unknown.