You wouldn't need to begin from anywhere less than T+0 to change an NFT. Once you have a hostile takeover you can simply assign its ownership to someone else, that the rest of the history is "immutable" doesn't matter (it actually isn't immutable anyway, you can change earlier stuff, it's just difficult as it would require a lot of computational power and deception to do it).

Also no, hashing doesn't confirm something isn't tampered with without a central authority. You want to know if you downloaded a file correctly, what do you do? You get the checksum the official server tells you is the right checksum. You only know it's not been tampered with because you have the "correct" hash to check against.

In decentralised blockchain, you can't know it wasn't tampered with because you don't know what the "true" hash is, only the one that is accepted by the blockchain. If a blockchain is taken over and old blocks are recomputed, the "true" hash is now the new one.

Yes, to hijack Bitcoin right now would be very difficult. You wouldn't need to run the hardware yourself though; convince dumb people to download dodgy mining software (that's nothing new) and boom, free money on top of it. Besides, Bitcoin isn't sustainable at all with its current energy usage, so the fact that it's hard to breach now doesn't really matter. It can't remain as it is if it wants there to be a planet left.