Try lowering your vcore in small increments. So if your vcore for example is 1.3V scale it down by .01 to 1.29V. Repeat until your system is no longer stable and go back up one notch. Also watch your load line calibration or offset. If scaling down even just a small amount makes your system unstable right away then you also have to scale back your clock (eg 5.0 to 4.9Ghz, 4.7 to 4.6, 4.4 to 4.3 or whatever it is) and that's what your chip can handle.
With regard to safe operating temps, the further you get from the throttle temps the better and 100C is way too close. I recommend keeping it under 80C. For longevity lower is better obviously. I can hit 4.8Ghz myself but then I'm in the 90s and so my everyday oc is at 4.4Ghz and my stress test loads are typically less than 70c.
Idle doesn't really matter but fyi mine range from 13-40c. Normal tasks for load temps are around 50c. It takes hours of gaming to get over 60c. This is what I've set mine at but you can get away with a little higher temps but you are right to be worried about hitting 100c. Hope this all helps! Have fun!