1- relative pronoun ( you could replace it with 'which')
It is correct and it means that eating less is the only known things that reduces the aging effects in lab animals.
2- taking care, not doing harmful things
3- A boss who doesn't support you, so that you feel unsure and insecure at work. The boss may be a factor in the suppressed hostility, but in the sentence, it isn't stated that way- you may feel hostile towards other workers, etc. In all, if you are angry, but don't express it, have little chance of finding another job and have a boss who doesn't care, you might have heart problems.