Is this too good?

"Return" - from where? From your graveyard? From the stack?

I don't think it's too good, but it's certainly exciting. When I read the first line I thought "Surely there's no way anyone can make a mythic Shock?", but you may have come close to managing it actually.

I'm guessing graveyard. At first I thought it was massively broken because I thought you exiled an instant or sorcery from your graveyard. (Funny how my brain assumed graveyard in both places because it was missing in one, huh?)

This is an interesting card, but it's definitely too good. Yes, people think of Shock as a mediocre card, especially since we got strictly better versions during Zendikar and Scars of Mirrodin, but it's a good card when you can bring it anytime you want.

Compare to Darkblast. Darkblast can kill a lot less creatures than this, can't hit the face, and is in a color that has much better removal anyway. Of course, Darkblast can get out when you want while this needs you to have instant and sorceries. Yet, the ability to do this more than once per turn is the absolute killer, because it's not a Shock anymore, it's a Lightning Blast or a Swelter that costs you an additional card.

Think of any concept that gives you this kind of versatility, like Dredge or split cards, and you'll see that the cost is upped from even the weak versions. Even if this doesn't turn out to be broken per se, it will always be that thing you wish you hadn't done because it's very oppresive.

I would say if you could do it once per turn, but the ability to deal 4 damage, sometimes divided, is dangerous. Something like or for 3 damage might be more reasonable, since the blowout option still requires six mana.

"A Lightning Blast or a Swelter that costs you an additional card" is right. But. That sounds like Lightning Axe to me, which was also 1 mana and common. (Bizarre when compared to Acceptable Losses, but that was just an awful card.) Admittedly it's a 4-star common, but still.

If Shock can be 1 mana and common, and Lightning Axe can be 1 mana and common, I find it plausible that this can be 1 mana and mythic. At worst I think 2 mana is definitely safe for this. Compare to the ludicrous power levels of many recent mythics.

I was going to do the Lightning Axe comparison, but hitting players is a big difference when there is recursion involved. I mantain my point that even though the disparate elements may look okay, the versatility as a whole is what pushes it too far. Yes, Shock is fair, Lightning Axe (maybe even for players) is fair, let's even pretend Swelter with a discard cost is fair. But having every single instant or sorcery turn into a Shock, which are very good in multiples, is certainly not okay. If a Cancel made every future instant or sorcery from my hand into a potential Divination somehow, it wouldn't be perceived as fair.

I know some mythics are ridiculous, but in many cases they are just powerful proactive things. When a removal spell is so good, it can completely negate a certain category of cards (think how Dismember forced everyone to play titans and Consecrated Sphinx). When burn spells are too good, they encourage racing in a non-interactive way. And, finally, your only chance against ever stopping this is exiling it from a graveyard, which most decks and colors can't.

As I said, a card can be very powerful and never be perceived as broken, but it can have big effects on the 'fun' of the game.