Overclocking is perfectly safe and it will not effect the life of your gpus (maybe your fans). Just keep your gpu cool and under 70C would be ideal.
Exactly. The only real issues that you can run into is the fans dying (usually a bearing) or the thermal paste/pad drying up and no longer being thermally conductive. If you notice a randomly large increase in temps with no change in your mining chances are it is the latter. A simple removal of the heatsink and replacement of the thermal paste/pad will resolve this issue for another few years. The board and chips themselves are essentially impervious to heat under ~90c