Thanks, Jimbo. But I can also imagine an alternative scenario, e.g. I am a whale and want to buy a lot of bitcoins without raising price to much, so I wait until there is a big sell wall, and voila, I can have my position without (much) slippage. I was not following prices long enough to know which scenario occurs more often. Knowing this would be one of the "crypto specific" trading skills, I guess

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I've definitely seen such a thing happen before but I would say it's generally quite rare. For the most part I find the whales buy/sell prices are similar to one another and so you often won't find someone looking to sell a large chunk at the same price as someone looking to buy a small chunk. At least that's what I've witnessed but this is more in smaller cap alts.
No. Walls in bitcoin have this reverse psychology. If one person touches it, everyone else will try to fill it.
I've also witnessed this quite often. It's hard for prices to approach a wall but often once that wall starts to get eaten it will be destroyed because people believe that once that will is down the price will go much higher/lower.