Now these are better questions. :)

As far as hydrogen goes, the building itself was not designed to be the containment (instead there's a hefty steel containment around the reactor and primary piping from what I understand), so I don't think they bothered trying to make the building resistant to explosion from the inside (since they didn't have to). It would still be better to have the building in place, but I think the answer is as simple as it wasn't necessary for the meltdown casualty analysis.

I'm not sure if the Mk I and II containments contain a core catcher or not. Modern designs do from what I understand, but I would imagine the Mk I does not. Perhaps the Mk II does.

The Three Mile Island core suffered much greater fuel melting and still didn't make it past the reactor vessel, but a lot of that depends on core geometry as melting progresses, and the TMI reactor vessel was much stronger than the Fukushima plants since TMI was a pressurized-water reactor. On the other hand, the Fukushima designs have the steel containment that would catch any core leakage out of the primary initially and as long as they have seawater available they can keep the steel containment intact, and without having to send that seawater into a high-pressure system.

I'm not sure what they mean by damage to the suppression pool. The suppression pool is a torus of water that steam from the primary system is ducted to when pressure relief valves lift, which acts to condense the steam and help with filtering some of the radioactive contamination to keep it from going airborne. Doing this heats up the suppression pool, which will itself boil eventually.

I assume this is what they meant when they talked about venting the containment (as opposed to the reactor or primary). Anyways, if the suppression pool is damaged I assume that means there is a leak of the water inside of it, which would leak to the bottom of the steel containment. Without water in the suppression pool any venting of the reactor would not be immediately condensed, and would cause higher levels of airborne radioactivity inside the containment building for Unit 2 (intact AFAIK). This would not affect the ability of venting the reactor to lower pressure though, which is important for allowing water to be forced into the coolant system.

Anyways, some of my IRC buddies have clued me more into a blast at Unit 2's suppression pool? I'll try to read up on that. Until then:

According to http://rushi-langaliya.blogspot.com/2011/01/boiling-water-re... control rods are inserted into the core from the bottom. I'm not certain of BWR design but obviously if all the metal is melted then the control rod could fall out (this was actually a concern with Chernobyl which used metals with very low melting points to hold up control rods). I will say the plant I was on had about 5 different ways to keep control rods in position whether there was power or not so I'd be very surprised if the control rods weren't latched in place now at the very least. But I don't know for sure, don't want to tell you otherwise.

* I don't know what their control rods are made out of, but the good assuming is that control rods melt after fuel does. That actually means the fuel would melt down away from the control rods before lumping up. If there is coolant there, the fuel shouldn't melt further. If there is no coolant, the nuclear chain reaction can't go critical again since the fuel is not enriched enough (the coolant actually also helps aid the neutron chain reaction). I can't speak to reactor protection analysis for BWR to that level of detail, but again I'd be very surprised if that's something that wasn't considered at some point between the 1960's and the last couple of years.

If they had considered it and concluded there was a risk of re-criticality they would have taken measures to preclude it. That might be why they're already using boric acid for instance.

As far as plutonium, it has a fairly long half-life, which means it's not very radioactive. With that said it is an alpha-emitter so it is very important that it is not inhaled. As long as it's not inhaled it's not a large concern from there radiologically. As with any other heavy metal though, it is highly dangerous chemically.

Hopefully better information gets put out at the press conference.