I doubt there is a non-arbitrary stance on this issue (or on many, many other ethical issues). It's not an issue of coherence. My view is coherent, just not non-arbitrary.
Even if a preference for happiness over suffering, or non-existence over suffering, is arbitrary (I don't think that it is) I still think that your position is self-contradicting if you make a fundamental distinction between small pains and larger pains, different degrees of the same thing, or view happiness and suffering as asymmetrical in value.
Maybe not all pain is bad. Some small pains might not be bad. This is more of a holistic, abstracted viewpoint, as opposed to, say, viewing each molecule of pain signal as inherently bad.
The difference between a large amount of suffering and a small amount of suffering is a difference in quantity, not quality. If suffering is intrinsically bad then the property of badness doesn't emerge at some arbitrary point between mild pain and extreme pain, it's intrinsic to the smallest amount of pain. An increase in the amount of suffering a person feels doesn't change the nature of their experience, it changes the intensity of it. Again, even if you reject the idea that suffering has objective intrinsic dis-value, if you regard suffering as bad then I think it is incoherent to not regard mild pain as mildly bad. Mild pain has the same nature as extreme pain which is why we still call it 'pain'. I don't understand how your position is holistic or abstracted. I don't know what that's supposed to mean.
As for happiness and suffering being asymmetrical in value ; happiness and suffering are antithetical. Happiness is negative pain and pain is negative happiness. Good and bad are antithetical just like up and down or dry and wet. If you regard suffering as bad then I think it's consistent to regard happiness as good because they are opposite in nature. Strictly speaking the absence of pain isn't good, it's just not bad. Negative utilitarianism denies that there is a such thing as 'good' but how can you have a concept of 'bad' without one of 'good'? The idea that suffering is more bad than happiness is good seems as unintelligible to me as the idea that X degree of cold is colder then the equivalent degree of heat is hot. I think that a hypothetical person who has experienced very little pain in their life, who has an unusually strong disposition toward positive emotions and a weak disposition to negative ones, might intuitively feel that happiness is more good than suffering is bad because they don't have a vivid understanding of how intense suffering can be. I think it's a lot easier to cause someone a devastating, shocking amount of pain than it is to make them extremely happy, there might be evolutionary reasons for animals to feel pain more intensely than happiness which could explain why maximizing the happiness of well-off people seems supererogatory and trivial compared to alleviating pain.
Also, I think you mentioned somewhere that different painful emotions aren't commensurable? What all negative emotions have in common is being inherently averse and dislikeable and I think you can compare the intensity of boredom or humiliation with depression or frustration in the same way that you can make trade-offs between pain and happiness depending on how much of either is felt.
I may pick some cutoff point between a dust speck and, say, depression.
I would also make an arbitrary cutoff between some pain and a greater amount of pain when it comes to what I think of as mild pain versus extreme pain but the very nature of the thing (and my attitude toward it) doesn't change when you change the amount of it.
How we compare them is always an ethical judgment call. (I agree the loved one dying matters vastly more, but this is an opinion.)
It's not an opinion that the grief most people feel when they lose someone they love is more intense than the mild pain caused by a speck of dust in the eye. If you regard pain as bad then you would have to regard it as worse (not necessarily the event but the pain caused, maybe there are some hypothetical people who could be really devastated by specks of dust in their eyes).