A technical question: isn't owning the majority of the CPU power enough to impose a malicious chain, regardless of the size and age of the network, and the consequent difficulty ?
No. Rewriting old blocks requires you to generate them again. So if you want to go back 6 blocks, you have to do the work required to generate them with the current difficulty
and continue to compete against legitimate generators.
I'm sorry but I don't understand your argument. The goal of the attacker is to harvest goods/services and have 0 BTCs at the end of the attack, but be plenty of goods/services. The attacker will therefore not suffer from the FRN/BTC ratio plummeting because of panic triggered when the the community realizes to have been hacked, which is after the attack is completed. I hope you agree with me.
That's much more difficult. A future version of Bitcoin will probably let the second recipient identify this attack immediately, since it is easy to see. A more likely attack is one where the second spend is back to the attacker.