Of the 4 things you listed, I don't really see a major issue. At least not one that I would lose sleep over.
1) Yes, the lever is spring loaded. The spring holds it in the locked position when closed, but even without the spring, the valve would still have to be turned to the open position. If the spring were to fail, the valve "could" theoretically open by some strange rift in the time space continuum (or a limb). Also, if you use the snap on lock, the a broken spring would have no effect.
2) Crud is a possibility, but if there was sufficient crud to foul the valve, I'd think there's more problems inside the engine than a slight oil leak is going to cause. This scenario also would need to be blamed on the individual using the valve and not verifying a proper seal. People leave the drain plugs out all of the time and end up with a floor full of oil. Is that the plugs fault?
3) A definite possibility. Again the lock should keep this from happening, but you should always check things like that periodically too.
4) I've heard the argument about incomplete oil drains too. Depending on the thickness of the oil pan, it's likely that the threads could protrude into the pan a bit and allow some oil to remain. I can't speak for every pan, but in the case of most of those I'm familiar with, that amount of oil would be pretty minute. Maybe a pint at the absolute most. That's roughly an 11" x 11" x 0.25" volume, not many pans have that kind of surface area at the bottom. I doubt the threads would protrude 1/4" into the pan in most cases. As you said, this can be rectified by removing the valve after a few changes or by trimming the threads of the plug back to the depth of the threads in the pan at installation.