I didn't quite have the time nor energy so late last night to digest all the gonzo from Greg Leal's site. This morning I'll figure out the Enceladus / Titan stuff.
Leal, excitable boy as he is, believes that Mars can be terraformed. I, less-excitable, counter-think that Mars will start on the Trade With The Rest Of The System model; such that the Ring Of Iron will be in place... rendering the equatorial surface even colder. With that ring up, Mars further has a magnetic field; by equilibrium, if Phobos doesn't bash up that Ring Of Iron in the meantime, Mars will get an atmosphere... someday.
Leal cannot wait and, to that end, would assuredly agree with Casey Handmer on big solar heating-mirrors at SML2 or (I'd prefer) the Mars / Deimos L4 and L5 haloes. Leal, also, wants more nitrogen - NOW.
Looking afield, he'd drink this chilly milkshake from the Saturn system. Why not Venus, which also has double Earth's nitrogen and isn't using it? Because he can't, duh. But what if Venus were orbited at 900 km altitude by a ring, which further can suck up Venus' volatiles...?
The question then becomes: delta-v budgets to get from low Venus orbit to a Mars-intersecting Hohmann, then Deimos; versus the budget for Titan orbit to Saturn escape to that Hohmann, then Deimos. For that here's the subway-map, admittedly Earth focused. If we jank around Earth, Venus LEO to Earth/Mars capture delta V should be about 4 km/s. It takes 4 km/s just to get from Titan LEO to the Earth-Saturn capture and then another 4 km/s for that Earth/Mars capture. Taking Earth out will improve both numbers. It won't improve the difference between those numbers. It is easier to get from Venus to Mars than it is from Titan to Mars.
Just getting the infrastructure to the Saturn system will be painful. I mean, some of the pain can be mitigated in that Saturn is so far that a NERVA engine becomes practical, maybe even Zubrin's saltwater torchship. But unless I miss my guess, there isn't much to mine out there; whatever metallics they use, we must export from the inner worlds.
tl;dr - if / when Mars demands nitrogen, the Martians will demand it from Venus.
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