Spinoff technology to fields other than space and space exploration?
Perhaps. Unlikely that anything needed for rendezvous with a comet would have commercial spin-offs, but there may be some materials improvements that trickle down.
Better understanding of the Solar System
Sure, that's a great thing - knowledge is good, and if this is purely in the pursuit of scientific knowledge, I'd be okay with that too.
leading to the human race getting a colony *somewhere off this rock* in preparation for the next Earth-bound major extinction event?
Now you're just being stupid. Is the human race going to live on comets when we get "off this rock"? Orbital mechanics are well understood at this point, so rendezvous with an object traveling through space isn't really advancing the knowledge of the human race.
Yes it does. It helps understand the mechanics and logistics of landing over a comet (such type of landings are not just ruled by orbital mechanics alone.) When we learn to reliably and predictably land over such objects, then we open for ourselves the opportunity to mining them. There is a shitload of ice and building materials on those floating monsters. Not only you open the opportunity to mining them, but to alter their trajectories, or even hollow them up and spinning them to get 1g.
Imagine you can take a 4km by 4km comet like 67P/Churyumov–Gerasimenko, alter its orbit and park it. Then carve it from the inside out, pilling out the dug out material, melting it. Filter it, separating it into portions of pure water and sludge then mixed with dust, or regolith, synth metal, ceramics or whatever that, when frozen becomes get a robust pykrete.
Rinse and repeat. Pure water = inner layers, pykrete = outer, protective layers. Rinse and repeat until you get an ice station with an embedded 4km by 4km cylindrical space in it, covered by several kms of ice.
Spin it till you get some decent g forces. Rinse and repeat, we have millions of floating icebergs on the solar system once we figure out how to use fusion. Fusion + a shitload of Hydrogen == no longer a need for depending on solar power to colonize beyond the belt.
Sounds impossible? Well fuck, we landed on the moon, we developed nuclear power and that just a mere century from the time of the steam engine. It was impossible to cross the seas, but people did it. Hell, even prehistoric man with nothing but hides, bows and arrows ventured into the Artic... and conquered.
Technologies advance, and if we really had the motivation, shit like what I just described could be done within decades. For what stop us is not so much technology, but economics and politics.
Barring a fundamental revolution in our understanding and knowledge of physics, there is no practical way we're ever going to cut the cord between Earth and even the remotest colonies in the solar system. As earth lives and dies, so die our colonies inside the solar system. And fundamentally, there's NO way we're sending a manned mission to another star unless:
What a load of cock. Colony survival does not depend on orbital mechanics or anything of the sort. It would depend on the state of technology. And there is nothing to prevent technology to advance to the point of making self-sustaining colonies.