And might those ethnic Russians have been there due to a sustained campaign on the part of the Soviets to depopulate the region of ethnic Ukrainians and replace them with ethnic Russians as part of a plan to increase loyalty of the region to the central Soviet authority?

Literally the exact opposite happened. The Soviets did a Ukrainization campaign to support and popularize Ukrainian language, literature and culture in regions where it had previously been marginal at best. Ukrainians were disproportionately represented in the leadership positions of the Soviet state. The Soviet Union was extremely pro-Ukrainian.