The giant, lightweight storage tank is probably going to have a lot of mass because of how big it is.

Well, metallic hydrogen is not necessarily a cost-effective way of transporting hydrogen specifically, as opposed to transporting fuel. You have to put a lot of energy into compressing it, then release that energy when you want the hydrogen back. And power is actually pretty cheap AF10 - everybody and their uncle has a fusion reactor, so mass doesn't matter that much in cost of transport. Then again, that means that metallic hydrogen might be okay after all... As far as mass/volume goes, large volumes mean you need bigger (hence heavier) tanks and it puts the structural forces in really weird spots during maneuvers because so much of the mass is out of line and can apply torque. Generally you want everything as tight in as possible.

Maybe they're not importing ice comets for the oxygen, MDFification, but for the hydrogen. As I understand things... The sun is a fan-fucking-tastic source of hydrogen, but as it's not conveniently bound to oxygen, transporting it is a bitch even without the whole fact that you'd have to somehow get it out of the sun and then pay the energy budget required to get it from the sun to Luna. Also, long-term ecologists will complain that every kiloton of hydrogen you remove from the sun is shortening the time until stellar collapse. In short, it's not economically viable. Mercury has some water to exploit, in the permanently-darkened northern craters, but I imagine the locals who live there are entirely uninterested in selling it offworld. Otherwise, it's not a source of hydrogen. Venus is not a source of hydrogen. Earth is an absolutely fantastic source of hydrogen and water, and would be perfectly positioned to supply Luna. It's just too bad about all the TITANs. Luna has ice in its own right, mind you, so it's not like they're without sources, but they want more, for reasons I hope are obvious. Mars could supply water, but they need all they can get for their terraforming; as they're currently actively importing the stuff, they're not selling a drop. Main-belt water iceballs like Ceres have loads of water and ice to export, but they're too busy supplying their local needs to think much about exporting in mass quantities. Jupiter could export hydrogen, but that would mean dealing with the Jovians, who are, well, to put it mildly, gigantic jerks. Sure, you'll deal with them if you can, but you don't really want to. Titan and the other places in the Titanian system are busy servicing their own local needs, too. They can export some, but Titanian military planners are probably factoring in water/hydrogen's strategic applications when they consider who to sell to/trade with, so they may be intentionally throttling back the exports heading sunward. Then you get to the far-out places, the farther the better. Really, think about it; bringing comets and stuff in-system only makes sense. It takes a while, but you don't have an urgent, imediate need. Very few people currently live out there, so there's nobody to complain or object if you take a comet or an asteroid and set it on a course for Luna. Sure, you'll probably lose some of your mass to whalers in the belt, but you'll still be receiving the vast bulk of an ice comet. Then you can just insert it into lunar orbit, maybe attach it to a space elevator and carve it up piecemeal, or even deorbit it intentionally, bringing it down lightly (as these things go) and picking it up off the regolith. Or, as was mentioned, you can divert it to drop it on Earth, if you happen to belong to the subspecies [i]homo moronis sapiens[/i].

IF they ever figure out the pandora gates they could empty oceans into sol system :P