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The measurement of TOI of a DUT becomes increasingly like a SAS obstacle course. Ultimately, there must be a middle ground, otherwise the strict, enveloping-pushing requirements on spectrum analyzer, combiner, signal sources and diplexers are so demanding that it means total surrender
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It's often an iterative process, I start out with the basic setup I showed earlier and then tweak it as required. I've got lots of different combiner types here, lots of filters and lots of attenuators including 1dB step attenuators. Some sig gens are better than others. I prefer to use a generator that can turn the ALC bandwidth right down.
The analyser's internal IMD should ideally be 20dB lower than the DUT and this can make it harder to achieve a working setup if trying to measure DUT IMD at -75dBc for example. The analyser IMD (and the sig gen and combiner IMD) has to be lower than -95dBc in this case.