fx 8320 max socket temp

In general below 70 C would be optimal for the FX-line CPUs. Beyond that instability may become an issue, especially if the CPU is overclocked. The motherboard itself should be able to handle higher temperatures, so you probably don't need to worry about damaging the socket. The CPU should start to crash and throttle before that happens.

62 C / 72 C are, I think, the numbers FX-owners should generally try to stay below. You're on the limit with the cores and a bit over with the socket, but if it's stable in P95, it might be fine. In normal use the temperatures should anyways stay well below those. However, if this was my rig, I would try to set up a small fan to cool the socket, either from below the motherboard or so that it blows air over the socket towards the VRM (if this makes any sense with the cooler the CPU has).

If you get a tower cooler, you may need to make sure the VRM gets extra cooling. Also remember to check your motherboards documentation. Some may "require" a top-down blowing design for CPUs like the FX-8xxx and choices are pretty limited if you want to get one that can handle the TDP...

"Stable in prime95" is admittedly a slightly vague statement. Personally I've considered four hours stable as stable. Initially I tested for eight hours, but that started to seem excessive. The problem is it's stable until it fails and there's really no telling when that happens. In principle you could run it for a week without issues and then have it drop a few workers a few hours after. Would that mean that the system is unstable? Probably not in a meaningful way (unless you're testing a server that will see heavy workloads). Now, I don't think testing for fifteen minutes is as good as testing for eight hours, but in the end it's about finding a balance that you're happy with.

I don't know any absolute max temperatures, only that the often recommended numbers in the overclocking community are 62C / 72C. Go beyond that and it's more and more likely you'll have issues with stability.

The VRM is on the frontside of the motherboard and cooling it from behind will do little good. The socket on the other hand can be cooled from below. The Hyper 212 Evo is a tower cooler and these are generally not great for generating airflow over the VRM, especially if they're oriented so that they do not blow air towards the rear of the case. For cooling the socket and the VRM, a top-down blowing design is best (such as the Noctua NH-C14S), but these are often rated for lower TDPs than tower coolers. This is why overclockers tend to stick with tower coolers and sometimes resort to adding small extra fans for the VRM and/or the socket.

1. There is, but it depends on the used components. It should be over 100C in any case. Your motherboard does not have a VRM temperature sensor, so your only way of finding out its temperature is measuring it yourself.
2. It was me (I'm the only one answering this thread) and I was referring to the socket temperature. 78 C should not be enough to damage the socket and if your CPU remains stable, then I don't think it's a huge issue since in normal use temperatures should in any case be lower. However, if it was my rig, I would try to keep it under the 72C more experienced overclockers recommend.

By the way, have you overclocked your CPU?