DC/DC overvoltage problem
I don't think this is an appropriate DC-DC converter to use when you have only 10% to play with between nominal and absolute maximum. A better converter may guaranteed the voltage to remain below 5.5V but this one does not, even with a 10% minimum load it can be 10% above nominal (right at your absolute maximum) and then add line regulation.. 

The 8.0V problem is due to insufficient minimum load, but even with the minimum load I think you're too close for comfort (it's caused by a simple circuit in the DC-DC converter in combination with leakage inductance in the transformer- and no opto feedback loop at all - semi-regulated by looking at the reflected voltage on the transformer). 

If you want to use this DC-DC converter perhaps you could reduce the output voltage to 3.3V or 4.0V with an LDO regulator and run the op-amps from that (watch the maximum input voltage on the LDO, many are only good for 6V). That would also probably give you better results because the LDO will remove some of the noise on the DC-DC converter.