Should this be a Misleading Motes?

I was having trouble remembering the name of cards using similar effects and my attempt to search for similar effects didn't go well. I was able to get Spin Into Myth and Whirlpool Whelm, and went off of the wording there. It does seem that the current use gives the owner the option instead of locking the card's owner to the card on the top like those older cards.

Yep. Forcing the card to the top prevents the game state from advancing and keeps the opponent from seeing new cards, which also keeps them from finding solutions to soft locks you could build with this kind of effect.

Before WotC came up with their solution I made up a handful of cards that made up for the difference e. g. "Put on top, that player may mill N" or "Put on top, that player may scry N" (which with N=1 is a long-winded way of doing WotC's wording, I suppose).

Adding "effects" to another spell is quite subtly complicated. Couldn't you just give this "Whenever you cast a instant or sorcery spell, you may <insert flashback reminder text>". That's well-trodden ground.

That would allow the spell to be cast, which means it's independent. That's a different flavor than what I was going for. But, there are numerous issues that could I arise that I did not anticipate. The penalties far outweigh the flavor I was going for. You are right. I will change the mechanic to your suggestion SecretInfiltrator.

This appears to just be splice from graveyard, which should be supportable in the rules just fine. I'm not sure what unintuitive corner cases might come up