We do not know how much people admixed until we examine the data. If we look at the R1a-M417 sample from circa 4,000 BC, for example, it differs little autosomally from the R1a-M417 samples in Corded Ware 1,500 years later. I would suggest this indicates that extant M417 admixed very little during this period, just as its females seemed to admix almost exclusively with M417 men. This is surely a more likely explanation than that it changed autosomally through admixture with Sredny Stog and Yamnaya before changing back to pretty much the same autosomal mix that it had before.
Sredny Stog and Yamnaya look to me like red herrings. They had different autosomal mixes from each other, and neither of their Steppe DNA mixes match the mixes in the Steppe components within BB or CW.