“The FBI was keeping tabs on Oswald, even weeks before the assassination.”
Evidently not close enough tabs. And maybe not a close eye on purpose..... Ponder this- if the fox is the one doing the watching, then the weasel goes into the henhouse, nobody will see the birds buying the farm. It wouldn’t matter if the weasel was compromised. Besides, the fox covered his involvement- enter Jack Ruby....
It’s all too hypothetical. We have a pretty good idea nowadays of how far the Soviets got infiltrating us, and it was bad in a lot of areas, but not that bad. Besides, even if the Soviets could have managed to arrange all that for Oswald, and somehow pull it off despite all the red flags that came with his defector status, it doesn’t answer the question of “why bother?” We know they had “cleaner” sleepers who could have done the same thing, with less resources required, and there wouldn’t have been a giant trail pointing back to the Soviets, as there was with Oswald. Just doesn’t make sense from that perspective, too much exposure beforehand and after the fact.