Your reputation precedes you, so I'm a bit hesitant hereHopefully the reputation is not for flaming and invective, I'm a rational guy and seek out contrary points of view to understand them.
1) Humans are a huge energy transformer on the planet, so by definition, they have a big influence. One can argue that by raising sheep (flatulence jokes aside, a source of methane) and killing whales, you can link us to pretty much a lot of change. My original point is that the world is a closed system, our existence as the dominant species on the planet ensures our place at the top of the influencer pile of all other species.
The primary cause for the disparity of our impact is that we use energy to amplify our existence. We use gas to move our cars, we use electricity to power our appliances and light our buildings and homes etc etc. To achieve 'neutral' impact with the rest of the species we would have to live a feral existence without fire. (Long way of agreeing that our species has more impact than other species)
I don't quite get what you were driving to with the sequestration question. I tend to think of the world, at a gross level, as a thermodynamic system. Heat goes in from a variety of sources and heat goes out through other systems. Current climate models highlight the reduction of heat loss through the atmosphere as a factor in raising the mean surface temperatures. This is also the primary argument in the IPCC work for the case of anthropogenic contribution to global temperatures. I find the argument credible.
"If you accept as a given that man-made CO2 production is causing climate change, would reducing CO2 emissions be an equally valid strategy that geo-engineering?"
I think its more nuanced than that. I think of "climate" as the name of the emergent property of 'weather' which occurs as a result of the interaction between a lot of variables in a very complex system. The planets climate is thus the measurable combined effect of all these variables. In all of the models, the effect of CO2 concentrations is perhaps the greatest single factor on the overall climate.
That said, how the climate "should" behave (which is to say predicted to behave) at various concentrations, has not been all that great. Models say Greenland should have less ice than it does, or more, or temperatures should go up by X but instead they go up by Y or not at all. And all that says is that the climate is a complex system that we don't completely understand it.
And because don't understand it well we can't really say what will happen if we pull a few peta-tons of CO2 out of the air, or cover millions of acres (hectares) of land with solar cells, or replant millions of acres of trees, or remove them. Every push on the system causes it to respond, and that response can affect future pushes. That is what makes systems like this so amazing difficult to study. Very hard to isolate variables.
So with that in mind, my answer is that I believe that changing CO2 levels in any direction will push on the thermodynamic balance of the earth. What I don't have an answer for, and I have yet to see a credible argument for, is a solid statement how those changes will manifest, either way. At the extremes? Sure, but plus or minus 10%? Not so much.
And if you're thinking "But if we went back to pre-1900 levels of CO2 in the atmosphere wouldn't we reverse the change?" I'm going to disagree with that, a hundred years of higher CO2 and the system has adapted, take it all away all at once, and you don't know what adaptations will be wrenched away, even slowly we don't know. Consider the following.
There is evidence that bird migration is happening earlier as a result of higher average temperatures. This has happened over the course of decades, possibly centuries. As the migrations change other parts of the system also change, and the birds change as well. Now we figure out we can 'fix' CO2, pull it all out of the air and in under a decade CO2 levels drop and temperatures go back to what they were a century ago. Now the current generation of birds is screwed, they make their migration at the wrong time and freeze/starve to death. Not enough generations to evolve a new timing.
Clearly it's speculative but I am trying to illustrate that it has never been true that there is some lever that humans have their hand on where they could just 'dial in' the climate.
"If the human species is creating a preventable crisis"
I think this is a good way to frame the different ways of thinking about this.
One is "preventable crisis" where climate change is like pollution or small pox, we just make a few policies and regulations and put our minds to fixing it.
I think this is a comfortable way to think about climate change because it contextualizes it in human terms of past victories.
The other is "ignoring a preventable crisis" where climate change is like an earthquake or a hurricane.
This is the less comfortable way (but I think more rational) way of thinking about climate change. There are things we can do to lessen our risk (generate less CO2 for example on climate, not build on earthquake faults for earthquakes) but we approach it from the perspective of living with a changing system, and maximizing our quality of life in the presence of those changes.
p.s. If you want to be pedantic, it is hard to predict the local effects on our solar system from the impending collision with Andromeda, a more predictable 'bad' event is that the Sun will eventually run out of hydrogen and Earth will not survive its 'Red Giant' phase, but again, outside the timeline that humans perceive as a threat.