1.
They clearly do care when, since they repeat their actions without fail every 50,000 years. If they all just up and left, they wouldn't be stopping ALL LIFE from making synthetic life would they? Even if they are just "in another galaxy" they still aren't fulfilling their function of stopping intelligent life from building synthetics at all, they are letting it happen. Something that their programming will not allow.
2.
A sense of self preservation does not equal them doing ANYTHING to survive. You have a sense of self-preservation, however it would be false to suggest that you would do anything to lead to your own existence, wouldn't it? The Reapers cannot, I mean physically cannot, disobey their programming in any way, unless it is deprogrammed (as in your example of Legion). Such is impossible without malfunction, which is counted under deprogramming. For example, all living beings have self preservation. However, some beings go into situations that they knowingly accept they will die in. Therefore, we can conclude some beings with self preservation are capable of accepting death as a consequence and still fulfilling their own will.