Hi there,
YES, bitcoin network can last forever, also price can be manipulated even if mass adoption occurs. Because those who got bitcoin before 2012 can sell every time they want.
For sure bitcoin can go back to 20k, as it can go back to 2012 prices.
Yes, but the number of people that got the coin at that time is limited and also the number of coins. So, it means that at one point, the whales will not have the coins to manipulate with if they sell them all. And why would they? At some point, there will be no more coins to be minded, and then what? The market will be forming the price, as any market does. At at any market, buy and sell of the asset influence the price.