1) Debatable - You would have to consider severity and type of injury, technique and experience of the striker and type and extent of conditioning. A badly sprained wrist from a palm strike when hitting a punching bag or kicking shield or accidentally striking too far up the hand is a real possibility. I sprained my wrist once and I found that palm pushups were more painful than knuckle pushups at the time.
The several Ortho's I consulted on the topic did not think it too debatable. The knuckles are easier to break (and metacarpals), than the wrist/palm. Palm pushups were more painful because the force/push point was closer to your wrist--the injured part.
2) Comes down to technique as to which is more powerful and is quite subjective. The fist is a much more compact structure than the open hand and you can tension the arm a lot more with a closed fist. Striking with the pisform bone can easily lead to a glancing blow. The knuckles also spread out the force a lot less than the palm since there is very little padding between skin and bone on the knuckles as compared to the palm and therefore greater pressure is applied with the punch. One less joint to fail with a palm strike but the impact pressure is less for the same amount of force.
The pisform bone I described is just as compact as the fist, and even a single knuckle, there is also about the same amount of skin over it as the knuckles, some callous also builds after awhile. Glancing blow? Hmm, I've never experienced that, even with a lot of pad/bag/sparring work (and real encounters). On spreading the force out...again see pisiform bone. Also the palm heel in general (or palm edge), really isn't much larger of a surface area than a few knuckles. The impact pressure is not less, if anything, its more--again closer to the force chain, like mook said--see elbow.
Also you don't want to "tension the arm more" [sic]. The more relaxed the arm is, the faster it goes, the more versatile it is, plus it has more tactile sensitivity, and more force can be transferred through it. You just made another argument for me that I left out, thanks.
3) Yes, however closing the fist for a block prevents the fingers getting jammed and sprained by a kick or strike.
Sure, but with a tad bit of proper training on hand position and palm strikes this risk is easily mitigated, you shouldn't be trying to block kicks with your fingers/hands/fists anyhow, if you are, get a new teacher. Nonetheless the extreme comparable versatility outweighs this risk.
Thanks for the discussion.
Best,
G