Was it keeping Smokey out, or was it keeping him in?
If Christian was the one that told Locke he had to die to get the O6 back to the island, and Locke dying inevitably leads to the death of Jacob, it stands to reason that Christian is not speaking on behalf of Jacob as he claims to be, and is actually working for MIB. I won't go so far as to say that Christian is MIB, but at the very least is on his side against Jacob.
If the cabin was in fact keeping MIB in, the ash being broken could explain why he was able to keep to Locke's form; it was designed to imprison him and keep the extent of his full power in check. Being imprisoned within the cabin prevented him from fully taking on an individual's form, only being able to manifest as an individual after death. Being dead makes it easier for MIB to impersonate someone in his weakened state due to the imprisonment.
Flocke saying that he wants to "go home" could be support for this as well. MIB was imprisoned on the island and confined for some unknown reason by Jacob. More support for this is the fact that when Flocke asked Richard to take him to Jacob in season 5 Richard didn't hesitate to take him to the statue. They didn't go strolling through the woods looking for a dinky cabin. Richard knew he was at the statue, and Jacob could've simply been at the statue the whole time. And unless my memory fails me, it was Ben and Ben alone that decided to take Locke to the cabin to see Jacob, and we know that Ben didn't even really see Jacob in the first place. Who's to say that he ever even really knew where he was? And it's certainly not unreasonable to assume that Richard couldn't tell that Ben didn't know because Ben is a master manipulator and liar (and, in my opinion, Richard is beginning to seem like he knows a lot less than they have lead us to think...)
And to touch on a slightly different yet still related note: If you think of the loophole being more literal, it could really be a loop. MIB tells Richard Locke has to die. Richard tells Locke. Locke dies. MIB becomes Flocke. Flocke tells Richard to tell Locke he has to die...ad infinitum. It's an infinite regress, just as the existence of Locke's compass is.