Alright well for starts, you're right that there's so many variables that come into play that it is near impossible. But not flat out impossible. It's not practical, is the proper way to put it. The cost-benefit is so beyond us that there'd be no reason to consider.
Attaining the right mix [of atmospheric gasses] is not impossible. However, it is the largest issue that faces full terraforming. Terraforming works in phases. Atmospheric pressure is phase 1, and phase 2 is implementing filler gasses into the atmosphere. That is to say... we would need an inert gas, primarily nitrogen, to fill the air. Too much CO2 is poisonous. Obviously you need oxygen, but too much oxygen makes the planet a combustible nightmare. Nitrogen is also important for all forms of life. Getting the right mix is difficult. There's enough nitrates absorbed in the soil to make it 'possible', but the reality is that that would take millenia to accomplish. ~1000 years. Not something we're considering.
But that's okay. We know it's possible to make a breathable atmosphere much like the one here. It'd take a long time, but it's possible. We aren't concerned with that though. Making it habitable only requires atmospheric pressure and temperature. Indoor environments and supplied oxygen is not something that's hard to contain. Pressurization and freezing temperatures are something that completely limit any realistic settling on Mars.
As far as debris, that's part of my hypothesis. I've estimated that the optimal impact size is a 2-6km asteroid/comet. Anything bigger is likely to have adverse secondary effects, like popping up too much dust and blocking out sunlight and cooling the planet even further. It's also important to know that impacting is a synergistic effect. It alone is not nearly powerful enough to get the job done. It's primary purpose also isn't to create energy, surprisingly. Most of that would have to be done via solar reflectors and manufacturing of artificial halocarbon greenhouse gasses on site. Impacting, however, has a lot to do with local thermodynamics, and more importantly, importation of volatile greenhouses gasses present in the asteroid/comet such as ammonia.
That leads to your next point. Finding asteroids that are rich in ices useful for importing is very very hard. It's my (weak) hypothesis that comets would be much more friendly for the task. I'm not sure if you assumed asteroids to be better because of mass or because of proximity, but that strikes another point... it's a lot easier to convince an object to move from the outer solar system in than it is to move an object from inside at Mars, because objects in the outer solar system move slower. That means you'd require a much smaller change in momentum to move outer objects. The fact that they're farther away just means you'll need to be more precise and you'll need to wait a whole lot longer. But who's counting time for an already impractical project?
The core of the paper is actually about finding the right asteroid/comet in the right orbit and figuring out how to move it.
Ethics is a compleeetely different story. I tend to favor the idea of "if there's no life there now, it's our sandbox to play with." But there's a lot of other issues. Another major issue is solar radiation. Mars lacks a magnetosphere, so the sun would literally poison us from prolongued exposure. Lot's of things to consider.