ive said it multiple times now

you dont need to extend the confirmation periods,, for over a decade now services have always followed the guidelines that if someone is spending $millions to wait 6+confirms to deem it settled. because if you DO THE MATH(said many times)
to go back certain amount of blocks to rewrite those blocks and then catch up and over take the network with a certain amount of competing hashrate takes alot of that hashrate cost to achieve. and services can also delay the withdrawal of the funds so the attacker wont try instigating the attack until after the withdrawal clears to ensure when they finally go backward X blocks. it then becomes unworthy of attacking the network just to get a refund

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as for manipulating the market price.. the market price is based on centralised exchange market orders on THEIR databases.. not the blockchain. so run some scenarios nd realise the separation between blockchain attacks vs market manipulation and realise if your end goal is "market crash", that there are easier methods that could have better success rates at less cost, compared to your un-thought-out scenario of thinking a 51% attack will affect the market as much as you assumed without thinking

run some scenarios. and try thing out. learn how bitcoin works, learn the mechanics and economics of bitcoin and what affects what