No fan of Obama here, but I figure he has a little more access to information about who did this than you or I do. I HOPE he wouldn't retaliate unless he was pretty confident in his intelligence agencies' reports. I have trouble imagining he would retaliate against NK if he was pretty sure China or someone else did it. But I could imagine him needing to if it were required to avoid letting China know how much we know about them.
Given all that NK has done to us and our allies over the years, I don't think we owe them an apology even if they are innocent of this particular crime. We could take down their internet for a couple years and still come nowhere close to making up for all the American, Japanese, and South Koreans they have killed and kidnapped over the years. Or for the thousands and thousands of our fellow humans (North Koreans included) they have killed and tortured.