51% attack means To own the 51% of hashing power over all not 51% of btc
When a coin is POW, yes, but POS is different. POS is vulnerable to a 51% shareholder attack. Honestly I don't believe that this could be done. Good luck finding 7 million Bitcoin to buy.
You have it wrong. You don't need 51% of the total coins to do the attack. You just need 51% of the total
staking coins to do the attack(which is much lower than 51% of the total coins).
Thats why PoS is not as secure as PoW.