1) For typical temperatures: http://www.techpowerup.com/reviews/
. Pick the chip/card type you want, read as many reviews as needed. The data is there.
2) GPUs typically tolerate very high heat loads, anything up to 60-70 is no big deal for a modern chip. The rule is the same as always: cooler electronics last longer
. For example my shitty cooler and very efficient GPU start at 30 °C idle and peak out at 60-ish. All my cards have been roughly in the 30-80 ballpark, and the hottest, an EVGA GTS 8800 512, fried itself after just a few hours of gameplay when a case fan failed to start (because Asus mobo control was teeeeerrible back then).
2.1) There's no "safe" high temperature. Other card components suffer from the heat, and there's a lot of those.
3) 70+ °C is on the high side and may not stabilise (run a loooong burn test). It might be typical of your chip though. Consider a better cooler or more CFM if you want silence - or just gamble it. Bigger fan may help a little - if you can even swap one in - but ultimately it is about the heatsink's ability to absorb and dissipate the GPU's heat.
4) CPU cooler can radiate a little heat on the card or block air flow in a non-standard internal layout, but that's it. No major effect; it's moreso the card that will heat up the case air and impede the CPU cooler's ability to cool the CPU. GPU > CPU when it comes to heat production in modern systems.
5) Stock to Noctua I would swap in a heartbeat. The default CPU heatsinks are adequate at best, the fan is usually bad (my guess is this is amplifying the annoyance in this case). My wager is it will not help your card, as it won't be able to blow cool air on the cooler or exhaust its hot air.
6) Case fans are easy to experiment with. Put system under high load and run the case fans as high as you can tolerate (try around 800 RPM), then determine if the coolers are good enough (parts overheating or not). If the coolers can stabilise, you can lower the fans' RPM and test again.
It's all about TESTING. Individual circumstances and rigs vary too much for there to be absolute answers, but the principles of comparing data (which you're already doing, good job) and testing your setup for optimal performance apply.
PS. Run Afterburner's OSD and put on GPU temp and FPS. You can monitor performance as well as your risk of frying an expensive piece of equipment.
PPS. For that last leg of the Silent Path - acoustic materials. Bare metal is prime real estate for amplifying and reflecting noise sources.
Edit 1: Fixed an instance of RPM instead of CFM.