While I agree with you for the most part, the problem is that changing his blood so that it has less iron in it (or removing it completely) isn't something that I would imagine would require a huge amount of control, pain or not.
If he can change to Human and back on a daily basis, altering the chemistry of his blood is something I can't imagine being troublesome at all.
And on top of all that, when Polaris did his trick he was ridiculously amped.
Well DS's scan is more than enough for me to credit his with the ability to change the make up of his blood in general, but never underestimate the power of pain in regards to affecting what one can accomplish. Enough pain and you literally can't think about anything BUT the pain.
And even if Polaris was amped, he was able to mess with J'onn that much because his blood had even more iron than a human, and Mag's doesn't need an amp to mess with the blood of a human so there's no real reason he wouldn't be able to affect J'onn's either. It can obviously be argued that J'onn may be able to overcome such a thing, but it can be just as easily argued that Mag's would be able to at least temporarily disable him the same way Polaris did.
Cool. Personally I wouldn't want to wager one way or the other on a solo Mags vs MM fight because I'm no where near well enough informed on MM's abilities, but if someone put a gun to my head and made me pick a winner I'd probably pick Mag's. But it's not something I'd adamantly stand by or anything cause I know he's got some really high showings and hype to offset his lower end stuff.
I'm also curious about this since my reading of the solo MM is patchy and some time ago. Getting rid of the iron would require either removing and physically dumping it somewhere so he would be permanently chemically changed (or retrieved it later), or a subatomic transmutation of the iron into something else.
Both seem kind out outside what I've seen him do (I guess he might be able to phase the iron away or something). I thought molecular control was more like rearranging atoms in his body, perhaps even down to altering/creating molecules from atoms, but not actual chemical transmutation of elements. Never seen him turn into iron, promethium, pure water, oxygen vapor etc. or do stuff like Metamorpho.
As far as the fight goes, he is in serious trouble facing all of them together - they can attack him on every level mental, physical, energy all at once. Gauntlet style and with MM jobbing turned off he might find ways to scrape through.
Interesting scans. I'm trying to recall any other time I've seen him do something like that. I mean can he just change into titanium/inertron/promethium etc or pure water or oxygen gas? Making an inertron armor or body would certainly come in useful.
And yeah, against Marvel characters his speed would make him almost unstoppable. I can actually see him beating any of these guys singly on a good day, though Mags and Apoc could be very tough fights for him, especially Apoc.