As I was reading the rules for the archetype, it says that as you level, you reduce the penalty for weapons larger than you, up to and including -0. However, nowhere does it say that you can ever reduce the number of hands required to wield it (i.e. not enough surface area to wield a huge greatsword). It seems logical that the class would allow for using weapons such as this (within reason), but the rules don't say so. Can you simply not use weapons bigger than 1 size above yours?
Second question, unrelated: can specific magic weapons not be altered via enhancement bonuses whatsoever?
nowhere does it say that you can ever reduce the number of hands required to wield it
Correct.
The original intent was to allow you to wield bigger weapons that you normally could, but it was changed in publication to not do that. (See here for a house rule proposed by the author)
Can you simply not use weapons bigger than 1 size above yours?
It's the same as everyone else. You can wield a one-handed weapon that is one size category larger, or a light weapon that is two size categories larger. See Inappropriately Sized Weapons.
can specific magic weapons not be altered via enhancement bonuses whatsoever?
In PFS you cannot alter 'named' magic items. In a home game, ask your GM. It kind of falls under the custom crafting rule area.
1: It isn't able to do that. IIRC, Paizo doesn't want any "monkey grip" type of effects in Pathfinder.
2: Many GMs will allow enhancing specific weapons/armors/shields further, myself included.
the titan mauler has an ability that allows two handed weapons to be used one handed (jotun grip) which allows you to use great swords and great axes one handed, but they have to be in your size category (so TWF with jountgrip could allow you to swing two great swords)
The 3rd level ability, massive weapons, works mechanically (as in reducing penalties) like TWF in that when using huge weapons two handed it reduces penalties for doing so (but scales unlike TWF, boo)
So was jotungrip written specifically to prevent massive weapons to be used in conjunction with it? Or was it written poorly, meaning the weapon must be appropriately sized for you at level 2, but when you get massive weapons all bets are off? If so there so should be a line in massive weapons stating "this ability alters jotungrip, by allows one handed wielding of larger sized weapons" but it doesn't say. RAW the one handed, 2 hadn weapon needs to be a medium weapon.
RAI im not sure.