regarding the radio altitude and thrust reversers.
I seem to recall that a 737 landed and could not get into reverse as the plane "THOUGHT" it was still in the air. I think it was based on compression o f the nose strut (going from memory, excuse me if I am so wrong).
SO a smart guy figured out that if you were at 10' radio altitude for five seconds (or is it 10) you might actually just have a problem with the weight on wheels switch so this radio alt feature would allow you to use the thrust reversers.
It doesn't mean you SHOULD put the reversers out in the air, but just in case you are falling off a cliff and the WOW didn't work, you have a chance.
NOW the DC9 would let you put the thrust reversers out in the air, though you were NOT TO DO SO. SO the only safety aspect was the pilot not putting them out in the air.
I respected douglas so much as they actually tested their planes with thrust reversers deployed to see if you could still fly them. Boeing relied upon theoretical calculations.