Power draw depends entirely on the graphics card and there are many that pull more than the specified 75W. But I don't know if risers that use e.g. USB cables actually do deliver 75W over the USB cable. I always was under the impression that modern risers don't take power from the motherboard at all and use their power connector to deliver power to the GPU. BUt I might be wrong, a clarification on that question would be nice.
Molex-4-pin is safe if it's a well built adapter that uses original Molex connectors and thick enough cables. I wouldn't use flimsy 3$-adapters from China indeed.

I don't like these splitters as the 6-pin connector is designed for 150W and the 8-pin connectors are designed for 225W. So theoretically you could send 450W via a connector that is rated at only 150W. Bear that in mind. Should be fine for powering the risers but don't connect powerhungry GPUs to those splitters.