On the second point, not only was it 1/3 of the equivalent (and specified) 235 design, it was also 1/3 less than predicted for itself, if I'm reading it correctly.
I wonder if that was an error in the prediction, or a fault of the weapon/design.
I've forgotten most of the nuclear chemistry involved in producing Pu, but iirc it can be done with natural uranium and a neutron source. But then again, if you have those, and can produce plutonium, why not just use that? Maybe it'd be helpful as a filler if you've got some, and only limited Pu resources, or to simplify weapon design (Any idea if the MET was gun-type or implosion-type, sources being unsurprisingly hard to find)?
Anywho, in regards to the larger point, I discussed the issue in another comment, here: http://news.ycombinator.com/item?id=2723675
Basically, proliferation concerns transition entirely to the honor system once someone is operating any sort of fission based reactor, even one powered by Thorium. Since it is quite easy to breed Plutonium merely by placing natural (and readily available) U-238 in a high neutron flux environment. Simply remove your Uranium samples every 90 days or so and chemically separate out the Plutonium.