This could turn into a long reply as it is multiple questions.
The concept of a precursor / progenitor race has been broached by more than one franchise, however from my own experience probably one of the more resonating examples was
ST:TNG "The Chase" within which the basic humanoid form seemed to be a result of genetic seeding.... A concept which seemed to be borrowed by Stargate SG1 later (After Sci-Fi [now Syfy] ) purchased the franchise from Showtime regarding "Ascension" and to a more short-term context of humanity being the slave labor force schlepped through the stargate to many different worlds by other races.
Overall these attempts are "good" as they give some canonical reference and explanation as why most races encountered are variations of the bipedal form; yet it is also "unhelpful" in a broader sense since it spoils some of the immersion aspect of television since at that point most merely see the "alien" races as "human actors with makeup"
Candidly, Farscape / Starwars / Babylon 5 did a better job at representing a broader sense of "alien life" throughout their franchises visually since it seemed that The "precursor / progenitor" approach of Star Trek / StarGate seemed to mostly convey aspects of morality / compassion as opposed to the more pragmatic aspects of how genetic engineering / seeding at such a long-term level benefit the precursors themselves within any meaningful timeframe.
