I'd simply imagine that someone Naruto or Minato-like would simply make it: either someone incredibly brilliant trying to replicate the tailed beasts power, or someone who due to their unique circumstances of chakra and probably isn't good at rote memorization came up with it because hand seals are too complicated for them,heard that signless chakra techniques are possible and thus just tried hard and experimented hard enough to get it.

Personally I see Rasengan as an uncommonly hammer-like weapon in a world of scalpels, at least for a version of the setting like this one. Given that its purpose is to basically be super-destructive and is a big ball of chakra focused into destruction emulating something even more destructive, its purpose its kind of considered orthogonal to what most ninja value in a technique, which is expanding their flexible toolset of things they can do outwit the foe, and the best use something like the rasengan has is surprising people with how penetrating and brutal its power is when most ninja rely on finesse and planning rather than direct force.

As for Mitsuko well....its a valid interpretation I guess. one could see it that way. I personally wasn't going for it, more trying to explain what her viewpoint is since we haven't seen it much compared to Jirou's viewpoint whom we've seen a lot.

On an unrelated note: Obito saw enough of Naruto using the rasengan with his sharingan to copy it right? how many canonical techniques do you think Obito has copied? just want to know for something else, not this game