LED PCB bulb - how much cooling do they need, what is their maximum temperature
It depends how long you want them to last. The hotter they are, the faster they die; the cooler they are, the longer they last. 

Parts from reputable suppliers will have temperature specifications tied to lifetimes, and they will also have thermal resistance specified - cooling the plate to 70C does not mean the active area of the LED would be at 70C... - based purely on the specs (and lack of specs) I'm guessing these are more casually sourced (direct from China via ebay, perhaps?) and may already be quality control rejects to take that path.

You should shoot for a lower temperature at the plate. And turn it over if you want the heat pipes to work.

Some better specified ones I have to play with spec an absolute maximum junction temperature of 125C, but that's at a point where they are severely derated and about to expire. Maximum current allowed starts to drop at 65C ambient, which is a good indication that they should be operating below that temperature in a well-designed fixture. A somewhat higher end one I also have to play with increases the max junction to 150C. But every step from the junction to ambient has thermal resistance, and that raises the junction temperature relative to ambient temperature. Aside from shorter life, high temperatures also reduce LED light output.