Measure twice, cut once, then call a carpenter

Well, at least we don't have to live by these words of wisdom (yet). As Carmen's post suggests, we're looking at putting a new door in our house to separate the basement suite from the rest of the house. To our dismay, the size of the opening doesn't match up with any standard door sizes...

Now the only questions left to ask are why is this the case and how do we fix it? As for the why, we have two complementary theories.

1. When the previous owners laid tile in the kitchen, the thickness of the new sub-floor plus the thickness of the tile and grout reduced the height of the door from the bottom
2. Perhaps there used to be a door in this peculiar door-shaped opening and rather than remove the old door-jams, they simply plastered over them, reducing the width from each side and the height from the top

So now we are left with a few different options:

1. Exploratory Surgery to determine if there are door-jams inside the wall, and where exactly they are.
2. Do some creative construction to make custom jams to fit an already cut-down door.
3. Give up on the idea of installing a door and put up a bead curtain

And in case it wasn't clear from this post, any comments or suggestions on how to proceed would be greatly appreciated.