I see your point. However, I think you would have to least include identical in conjunction with duality working with sp/sx and so/sx. Perhaps, activity and mirror like you've mentioned are too different and not compatible enough to overcome conflicting instinctual patterns. The same goes with sp/sx and sp/so, you say it is only possible with duals or identicals, but I think this instinctual pairing has potential for within quadra including activity and mirror. A good example of that is Shaebette having a successful relationship with her mirror with the instinctual pairing of sx/sp and sx/so. This contradicts your assertion that only identical and duals can succeed with this instinctual pairing.
I think the main crux of the issue here is not necessarily the ordering of best relationship to worst for instinctual stackings, because even though I don't completely agree with it, your ordering may possibly be correct. Rather, the main issue is the limits you are are arbitrarily imposing for what can work or not work with these relationships. It is fair to rule out outside of quadra for relationship success for some stackings, but to do so for within quadra is stretching the impact that instinctual pairing has on relationships when Socionics ITR has a much stronger impact IMO.
I agree with you on this part, well said. I personally find so/sp to be the worst, I feel like I'm dealing with someone that is basically the opposite of me and communicating with them is very grating. I generally get along well the best with other sp/sx. So/sx, sp/so, sx/sp and sx/so seem to be decent for me, but not necessarily good or bad with different plusses and minuses.
Sx/sp is certainly possible for me, I wouldn't rule it out completely. However, I find I do tend to worry a lot more about sp over sx. Sx related matters is something that I enjoy, but I don't really stress about it in the same way I do about sp related matters. When it comes to so matters, I am at least more indifferent to it compared to most people.
I agree with you in the sense of sp/sx and sx/so getting along better than sp/sx and so/sx and the same goes for sx/sp and so/sx getting along better than sx/sp and so/sp, that makes sense from personal experience. However, I personally find so/sp more grating than so/sx in both short term and long term interactions. I just figured that sp/sx and so/sx would have a more positive relationship because they value sx about the same even though they value sp and so completely differently rather than sp/sx and so/sp where sp, sx and so are all valued completely differently.