This might get into some weird philosophical ground (though at least it’s a grey area as opposed to literally making a pact with Satan), but Spidey could have just taken the Supreme route and transferred May’s brain patterns into the body of a robot of some sort.
There’s precedent for this sort of thing with the Vision and Jocasta.
You’re shitting me.
Spider-Man is evil now?
No, just dumber than a sack of bricks. It’s almost as if Mephisto was trying to prevent Spidey from interfering with one of his plans. Before Spidey sold his soul his powers were greatly amplified (as was his emo quotient) because of The Other storyline.
My guess, Spidey finds a loophole in the contract somewhere and he gets his amplified powers back and saves aunt May.
What I wanna know is this: Where did Mephisto* get the power to break the heretorfor unbreakable rule that changing time just creates an alternate timeline? The Cosmic Cube couldn’t change time and, IIRC, neither could the Infinity Gauntlet. Mephisto has never had that kinda oomph before.
And if he DID have it, why did he hold it in reserve simply to bust up Spidey’s marriage rather than using it to capture the Silver Surfer (who he was after for what…20 years or so) or take over the world or something? Really, making a couple break up isn’t really on a SATAN-level evil. Hell, it’s barely Eric Cartman level evil. (Now if Mephisto made MJ chop up Aunt May, cook her into chili and serve it to Peter, THAT would be Cartman level evil)
*And why the hell is he called “Mephisto”? The name is “Mephistopheles”. It’s like calling a Greek water-god “Posdey”-it just sounds stupid.
Well He and Peter are really dumb. So it’s one idiot outsmarting another idiot while unaware of how stupid they both really are.
This proposed solution will be disqualified, but a couple universes over, there’s the giant Cosmic Eraser known as the Dragon Balls… 
…and Aunt May probably counts as important enough to the timeline that you could bully–er, I mean, convince Gaspar to make a Time Egg for her. Now all you need is a reasonably shaped facsimile to substitute in the one that got shot.
Ohh–if we’re allowed other universes, we just have the sole remaining survivor of a dying universe punch a big crystal wall causing ripples in reality such that Aunt May was never shot.
Hey, it worked with Jason Todd.
(For what it’s worth, I kinda liked (as a ONE TIME THING) Superboy-Prime’s reality punches.)
“You mean she wasn’t just getting senile when she kept almost walking into traffic? And dropping vegetable knives? And when she purchased a dozen bear traps on eBay? That explains a lot.”
Okay, I git a coupla more today…
•Find one of those brain in a jar life support systems, and, y’know…saw, scoop, plop.
How many brains-in-jars exist (in whatever state of mobility) exist in the Marvel universe, anyhow? Maybe they should form a team…that’d be awesome. 
•To deal with Mephisto, have Doctor Strange temporarily summon Conan to help him out (this is not wholly without precedent, even in Spider-Man books). Conan wouldn’t put with with all the existential wangst and metaphysical agonizing crap; he’d just stab something in the face.
Forget mere Silver Age nostalgia…let’s see how getting the day saved by a refugee from 1982 “ages” the setting. (Double )
If we’re going into alternate universes, then all it would take is a phone call to Wonder Woman to ask her to borrow the Purple Ray.
I’m not even sure that the DCU is non-canon to Marvel 616 – there have been a number of crossovers between the two. Some might still be in-canon.
Actually, more likely to still be in canon than, say, getting a replacement body for Aunt May from Baron Karza’s body banks.
There’s that Access character (heh, no pun intended), who’s owned by both companies, who specifically has the power to transport himself and others between the universes.
Bring him into the equation, and we open up everything from Lazarus Pits and Purple Rays to even more advanced brain scooping and cyborging (is Cyborg’s father still around?) technology.
Not to mention even more ridiculously convoluted mega-crossovers full of lots of Earth-shaking revelations, violence, and partial nudity that’d eat up an entire summer…but hey, really, what are the chances anyone’d go for one of those?
Are you saying they can’t change the past?
I think it’s inconsistent. Sometimes you have characters go back in time only to do what they were predestined to do in the first place. But others succeed in changing the past: E.g., Bishop succeeded in saving the X-Men from the traitor who had killed them in his timeline.
He didn’t actually sell Mary Jane to the devil. He (well, they) sold their marriage to him - history has been changed in order that they never got married and broke up at some point, in in return Aunt May lives.
Still quite immensely stupid and out of character.
In so doing, Bishop created an alternate timeline while his own home timeline continued to exist with dead X-Men.
Apparently, the exception is Mephisto–he explicitly just now did it–which is a huge gripe I have with this storyline (among many). If he’s had the “Change the past” power all along-which has always been ruled as a no-no that even Eternity can’t violate (IIRC, there was a storyline where past-changing was tried and the Living Tribunal had to get involved)–the question still remains "Why use that power to simply play “Let’s break my buddy and his girl-friend up” type high-school games?
Oooh, I’m on board for this. Any excuse for She-Hulk & the Magistrati to guest-star.
Marvel has changed its policy on time travel. The first Young Avengers story is flabbergastingly inconsistent with the previous rules.
Maybe someone blew up Gallifrey-616 (also not without crossover precedent).
Hell, technically, with the established crossovers, you could inject Aunt May with revitalizing G-cells, turning her into a 100 meter kaiju; or use a spark extractor transfer her mind into a a protoform, at that. (Rights issues might be a bit tricky)