Hey livvy, I'm sorry it took me so long! I want to preface this by saying I am nowhere near Todd and Stoika's level but maybe I will just notice different things...One is the DW Nessus aspects. Your Chiron on his Nessus, his Pluto on your Nessus with his Saturn nearby. It definitely looks like there was serious abuse between you in a past life.
His Jupiter trine your Sun/moon/IC/Juno(?) is such a wonderful aspect! He must really accept you for who you are. And then your Jupiters are sextile, how nice.
Your Neptune/Uranus near his moon must make him hard to read emotionally, no? Must be frustrating for a double Pisces! He has a lot to teach you on an emotional level. Pay attention to how he rationalizes and interprets his feelings. (My SN is in Cancer too, I think we are about the same age with the Neptune/Uranus conjunction! I have had a few people in my life with a Cap moon and their emotional maturity, especially in handling relationships and other people, has always been so impressive and illuminating for me. One in particular teaches me so much... This is assuming it is a mature Cap moon, but the immature ones can be helpful for what not to do!)
As far as your composite question about Mercury/Mars/Neptune... Let's first consider that conjunction. It's in the 12th house of all things hidden. Lots of confusion and just difficulty reading one another - HOWEVER, I would say that if you can find a way to access them then they can be very illuminating and deepen the relationship far beyond the average experience. With Sag on the 12th house cusp, optimism and teaching/learning (I mean from one another) may be difficult to access or at best subconscious. "There may be karmic issues with freedom or beliefs that need to be addressed, and preachiness or some arrogance may need to be let go of." The conjunction of these three planets can go either way... you can be open and communicative (Mercury) about normally hidden/secret and subconscious things, and be tremendously compassionate and understanding about what you reveal (Neptune). On the other hand, you may be driven (Mars) by subconscious motives and problems that are better worked through separately before coming together, and if there's not enough grounding (Cap) trust there you may end up keeping rather than sharing secrets (Mercury). Considering that these planets are all also in Capricorn but placed in the 12th house, I would guess that some authoritarianism or control issues, or perhaps more practical ways of communicating and relating, might create an obstacle to bonding physically, intellectually, and/or spiritually. My guess would be that you might get stuck communicating or acting on practical necessities of life without quite getting to the deeper, more psychological and spiritual topics that will really help you bond and understand one another.
In the composite, this conjunction is squared by Jupiter/North Node conjunction, magnifying any related benefits or issues (Jupiter) while making them inhibiting to the relationship's growth (NN). It is sextiled by Pluto in Scorpio, which seems to intensify it, but it should be beneficial since Pluto helps uncover some of those subconscious, buried things. Maybe leaning into Scorpio/10th house activities could help reveal some of the hidden 12th house topics?
Short version - I think you'll have to work really hard together and individually to become aware of and communicate about hidden or subconscious things impacting the relationship. The wonderful thing is that you have all the potential to do this and probably don't need to worry so much if you'll accept one another with compassion and understanding - that's a given with Neptune there. Plus, both of you seem to have pretty "selfless" placements natally.
Now let's consider the impact on your nodes...
I would think most of the subconscious issues impacting the relationship are coming from him, from his past - his childhood or past lives or family history. Helping him talk about and cope with subconscious burdens from his past is something you're likely able to do and help him work through. (Especially being such a watery, empathetic person!)
For you, with the conjunction falling on your North Node, helping him work through this and also working on how your own subconscious patterns affect the relationship, and essentially, taking responsibility (so Capricorn!) for your psychological baggage and any projections will help you grow enormously as a person. (With the same NN, I've found taking responsibility for my emotions and these exact things in relationships has been enormously important to the successes of both my relationships and myself as an individual.)
In my experience, reversed nodes are a wonderful thing. You can really help one another grow into more mature individuals if you look for the lessons and appreciate what one another has mastered. Again, pay attention to his moon (the ruler of your SN) in Cap (the sign of your NN). That's significant.
I hope this is helpful! Please let me know what, if anything, resonates and what doesn't.