Grexit would be much more manageable if they didn't go alone, or the opposite.
In the end you have to ask: what is Germany's objective in all of these negotiations? To help Europe as a union? If so the plan has to allow Greece to eventually come out as working in Greece's economic favor. It obviously doesn't, and Germany and everyone involved knows it. So that leaves us with the idea that Germany's position is not about Europe's interests but its own, in that case the union is already over.
In the end the fact remains, a union of countries in similar difficulties would work much better. The so called "southern Europe", which would really include Ireland too, would be much better off working together out of this false union that is just going to cause each country to handle a crisis on their own one after another. Whether Greece leaves or not, there is only one outcome, which is that the union will break down and end, but how much damage will be done before it happens seems to be what the negotiations are about and some are interested in more than less.