I agree that it depends on how the situation is handled, and that's exactly why it is crucial. But no need to quibble over that detail.
I also agree that it's possible tensions can be diffused if cool heads prevail. The likely annexation of Crimea and the rest of Ukraine becoming a part of the EU will mean some differences in the way business is conducted. Actually I think that if that happens, and Ukraine DOES NOT become a part of NATO, that might be a good thing, because it would mean that the issue would be somewhat settled and would therefore result in more stability, if both sides leave it at that. However, if there is an attempt, in the short term to bring Ukraine into NATO, that will be a problem and instability in the area will continue, with the possibility that the conflict could erupt into a global conflict. Having said that, if Russia is allowed to have Crimea, they should be able to tolerate, although with some discomfort, the rest of Ukraine being in NATO. That's only in the long run though, and if that's what the people of Ukraine ultimately want.
I believe that if, as Kerry as seemed to indicate, the US and Europe try to impose SUBSTANTIAL sanctions on Russia for the annexation of Crimea, then that will be a problem for both sides. As I stated before, Germany in particular gets close to 40 percent of it's natural gas from Russia. The imposition of substantial sanctions would almost certainly mean a disruption in those supplies. Although I think it will hurt Russia more, Europe and the US will feel a substantial bite as a result. Why? It would mean an increase in energy prices for Germany, who has been the engine of the European economy. German exports will become more expensive which will depress demand and the already fragile European economy will suffer significantly and likely go into an depression. The US will likely have to use the power of the Federal Reserve to prop up the European financial system as it did back in 2008. This will put increased stress on Federal Reserve which will decrease confidence in the global financial system, and could likely send the US economy, which is still struggling to recover from the effects of the Great Recession, into depression as well. Like I said, although Russia will likely suffer most, the effects on Europe and the United States will be extremely unpleasant to say the least. So I think substantial sanctions are in no one's interest.
But back to the issue of NATO membership for Ukraine. If there is a push to bring Ukraine into NATO in the short term, Russia will almost certainly respond by using what influence they have in the non-Crimean part of Ukraine to create as much instability as possible. And they may even respond by attempting to seize the parts of Eastern Ukraine where their influence is very substantial. If that happens, then things could get out of hand very fast, especially if Ukraine is actually admitted to NATO. That could lead to all out war between the US and Russia. Although I don't think this scenario is likely, it is not out of the realm of possibility, and which is why, as you have seemed to indicate, the situation must be handled properly.
The path of least resistance would be for Russia to accept leaving Crimea as an autonomous part of Ukraine for the short term, with the possibility that it could become a part of Russia in the future. In exchange they could receive the guarantee that there would be no NATO membership for Ukraine. However, honestly, if I were in Putin's position, I would settle the issue of Crimea now, and write off the rest of Ukraine. The US and Europe won't like it, but I think they would live with it, after extracting a painful, but bearable price. I think that is likely where we are headed.