> there is nowhere else within reach that is remotely similar to either planet

Now, hear me out here but have you considered Venus?

Cons:

  Runaway greenhouse effect, sulfuric acid clouds, crazy hot, etc
  Very long days (1 year = 1.92 Venus days)

Pros:

  Nearer to sun => better solar energy collection
  Nearly the same gravity as Earth (0.8-ish)
  Lower delta-v to get there from Earth (someone double check my math on that)

You'd need to get material built into a 'solar umbrella' to cool the planet down, reflect light elsewhere. One idea (Kim Stanley Robinson?) was too make mirrors that reflect light from the way-too-bright side and have it instead reach the 'dark for half a year' side.

Once you've got it cooled down, you're a lot of the way there. Bleed off some of that atmosphere (hey, maybe Mars could use it?), get some plants converting CO2 into O2.

It's ludicrous, I know, but I find most conversations about terraforming are. The only real advantage that Mars has is that we could live there while we work on it. Venus, not so much.