ok. Here's what I don't get.
I went through the recent threads and all say Jupiter in cancer is positive and a sign of interest but now you're telling me, there can be a twist and it isn't positive based on the speed of the planets.
no, Jupiter in Cancer is not "a sign of interest" as you say. interest is shown in other ways (application from one planet to another, reception, emplacement, etc.).
Jupiter is the greater benefic and when he is well dignified as he is in Cancer his power increases. if he was angular, it'd be an indication that the horary was prepared to bring you good news. if we was collecting light for your significators it would show a planet well disposed to help and assist. in this case he's neither angular nor collecting light.
when the significator of the quesited is exalted, especially in a relationship or contract chart, it shows someone puffed up, self-concerned. arrogance is a word we see frequently in relation to exalted planets. Lilly says that exalted planets show, "a person of haughty condition, arrogant, assuming more than his due" (CA, p. 102).
so is there power to do good? certainly. but it's not ideal to see L7 strong in essential dignities and L1 peregrine and out of essential dignities.
about the speed of the planets -- that's a separate issue, but still relevant. when asking relationship questions, there are only a few planet combinations possible.
Moon - Saturn
Sun - Saturn
Mercury - Jupiter
Venus - Mars
Sun/Saturn and Venus/Mars are good matches for each other. the Sun is the central source of heat and light in the solar system, and Saturn is the most superior of the planets. their is an equality in power and importance between them. same with Venus and Mars -- they have similar speeds, are both only 1 sphere removed from the Sun (Venus just inside, Mars just beyond). one is the lesser benefic, one the lesser malefic. again, we see an equality in their positioning.
when you see Moon/Saturn and Mercury/Jupiter as significators, straight away you know there's an unequal power dynamic. someone is definitely doing much more of the chasing, and that person is signified by the faster, more inferior planet. you don't want to build a whole judgment on this one point, but it's certainly very reliable and is a great place to start.
If Neptune on MC is of concern as a delusion, does it say anything that Sun happens to square Neptune while Moon is going to trine Neptune before it squares Mars?
Sun is separating from a square to Neptune as you say, and the Moon applies to trine him. I wasn't going to bring this up, because the outer planets are really small accents in charts as opposed to key players. but, if there's anything to get from this it's a triple re-inforcement of the Neptunian principle of illusion, confusion, not seeing things as they are. impacts from the luminaries promises disillusionment in time.
And doesn't Mars rule the 11th house?
indeed he is, but notice he also rules the 6th. I'm not sure why you mentioned his rulership over the 11th, unless it's to say that the Moon's square to him might be positive. it's anything but. look at how Lilly describes an ill-placed Mars: "a prattler without modesty or honesty, a lover of ... quarrels, ... of turbulent spirit, perjured, obscene, rash, inhumane, ... unthankful, treacherous, oppressors, ravenous, cheaters, furious, violent" (CA, p. 66). there's a tendency for traditional astrologers to overemphasize the dangers shown in a chart, but even the most conservative astrologer would encourage you to take caution in engaging with such a Mars.
If the Sun is important here, does it say anything if it is in the sign of Gemini, of which the ruler is my significator?
well, if it does it's probably not the overriding message in the chart. I think the important thing here is that the Sun is aligned with the Ascendant, has the power to bring the truth to light, and your significator is preparing to retrograde back to a conjunction with him (combustion).