Addressing just your first item, the purpose of the cockpit voice recorder is to provide a record of all sounds in the cockpit from the time the recording stops back to however long the recording loop goes for. It should be used for accident or incident investigation only. You don't want pilots modifying what they say (or passing notes or using hand signals) because they have to be concerned with usage of the recording for anything other than accident investigation. Pilots don't expect to crash, thus, secure in the knowledge that the CVR will not be looked at except as part of accident investigation, they are free to say what they will.

Some pilots are more paranoid than others, at least they used to be back in the 1990s. I occasionally flew with pilots who availed themselves of being able to erase the CVR after a flight. On 747-100/200 aircraft you could erase the CVR if the engines were shutdown, you were on APU or ground power, and the parking brake was set. You did it by pressing and holding a button next to the microphone on the pilots overhead console.

I'm aware of at least one third-world airline that used to use the CVR to check what pilots had been saying, not for training purposes but to find out what the attitudes of the pilots were toward the company. We were flying Hadj charters for them. One day one of our captains caught them trying to change out the CVR. He ordered the offending technician off the airplane, immediately notified our management of what had happened, and made sure every captain flying on the contract was aware of the situation. I was so angered that anyone would try that kind of crap that for the next several days if any of their people came into the cockpit, I in no uncertain terms informed them that by U.S. rules (and we were in a U.S. registered airplane having to operate under U.S. rules) the CVR was to be used for accident investigation, not for spying on pilots.