1) I think it was because the 5cm height is backed by a huge width, when crawling towards the land slope creates a very strong (not necessary high) wave that just keeps pushing forward.
Tsunami waves have an extremely long period. A good surfing wave forms when a swell 2-5 m amplitude with period 13-24s when it starts to feel the bottom of the ocean.
The amplitude of the tsunami wave in the open ocean is much smaller than wind-generated swells but they can have periods of 60-70 minutes so they are more of a surge that just keeps coming when they hit land, and as you say when they start to feel the bottom they stand up and the large volume of water in the wave makes it stand up larger and more powerful than you would expect for a smaller amplitude swell.
Every wave has a peak and a trough and the trough of the tsunami wave is what causes the water levels to recede so drastically before the wave hits.
Geographic features like valleys that get narrower and narrower will cause the same volume of water hitting the coast to make a much more drastic change in water level
Imagine the energy stored by lifting a kilometre-high column of water 5cm. Now imagine how high that energy can lift a 5m column of water once it reaches the coast. Obviously there is the 1/r dissipation because the circumference increases, but still, deep water lifts like this can cause a massive surge at the coast.
The amplitude of the tsunami wave in the open ocean is much smaller than wind-generated swells but they can have periods of 60-70 minutes so they are more of a surge that just keeps coming when they hit land, and as you say when they start to feel the bottom they stand up and the large volume of water in the wave makes it stand up larger and more powerful than you would expect for a smaller amplitude swell.
Every wave has a peak and a trough and the trough of the tsunami wave is what causes the water levels to recede so drastically before the wave hits.
Geographic features like valleys that get narrower and narrower will cause the same volume of water hitting the coast to make a much more drastic change in water level