1) That was a good answer. Just one comment to make: Once Keigoku does work (ie. hits a target it can affect), it dissipates. *Straight from the CC Submission* "This also lets the Keigoku pass through physical substances without minds, such as earth, water or trees to reach a target behind it, since it loses none of its powers on the non-sentient object." This implies just what I said: When it touches something it harms, it uses up it's power and stops, so it won't pass through a summoning or shinobi (remember this if your Tobirama ever fights a Nekura <.<).
2) Just one comment to make on this answer. Every time you make "a" portal, you actually create two: one at the source of the Keigoku, and one where you want it to exit. For the most part, the first portal is on the moon, and the second is in the battlefield. However, you can also create two connected portals on a battlefield (though this gets into complex Nekura mechanics, so we'll leave it for later). (You get to this in question three, but i didn't see it here, in the question about portals, so I made my comment <.<) But "portals" is basically all I was looking for.
3) Yeah, that's basically it. It's similar to a Jinchuriki seal; it connects our chakra to the Juubi's chakra, the seal itself originally being formed of Keigoku (see Clan History). Sending chakra to the Mark, yes, allows us to open portals and draw Keigoku through the portals. Because the CC Submission says "An unwanted ability is that of the Nekura's bodies, after or during a battle, being so filled with Keigoku that they reject any foreign chakra from entering their body", it can be said that the Mark channels Keigoku through our body as the bridge between the two portals, filling it with Keigoku that has a "sentience" of its own and can kick out chakra like a partner/Bijuu. It's a bit of a difficult concept, but that's essentially it; allows us to make portals and draw Keigoku.
4) That's what I wanted you to do; answer without using sources outside of what you know yourself <.< Now I'll tell you what I told L: Never word it like that again. See my announcement in the group; what it basically boils down to is "Do not say the Nekura are immune to Genjutsu as this ability goes much deeper, and this is a very shallow way of looking at the clan. You begin to touch on the "why" without ever actually saying it, so I'll answer. We reject foreign chakra because the Keigoku filling our bodies (see my response to "3") acts like a partner or a Bijuu to kick it out immediately. All foreign chakra. This means Natural Energy as well, barring us from things like Curse Seals and Sage Mode. We also are immune to things like the Gentle Fist and the Kiyoshi's "surging one’s chakra into another’s body through contact" (they have no techniques related to this that I'm aware of, but that sentence is taken directly from the app and Emp himself said we would be immune to attacks like that, so I may as well include it in case), but it also means that Medjutsu essentially has to work harder due to the fact that they need to do everything either physically or with extra chakra because we naturally reject the chakra being used (if it's injected into use. Using techniques like Chakra Scalpel to physically cut us is viable). Now, you already explained that we aren't immune to genjutsu in your explanation, so "Rejecting Foreign Chakra" is clearly the correct statement. And yes, we can open portals in free-form, but we can't draw Keigoku from the moon through it without a technique.
Things you missed would be the fact that Bijuu refuse to attack us. So if a Jinchuriki enters any of their forms (Red Chakra-Cloak, Incomplete, or Complete form), they give their Bijuu enough power over their body that the Bijuu will stop them from harming us; like the Edo Tensei used by Kabuto, the opponent can do whatever they want within certain limits (ie. Mu could fight however he wanted, but he was forced to fight against the Shinobi Alliance. In the same way, the Jinchuriki can do whatever they want, as long as it isn't intended to harm the Nekura). A Jinchuriki can still use Bijuu abilities (ie. bubbles, steam, sand, etc.) they normally can without these forms to harm us. You also missed our cloaks, which allow us to defend against all forms of freeform with minimal effort. The Drain is also technically passive, but you don't need to know that right now.
5) <.< At the very least I was expecting something like how our power scales with rank (that's actually all i was thinking when I made this question), but if you don't want to write anything, that's fine. Honestly, even "There can only be 4 Black Knights, 2 Clan Council Members, and 1 Clan Head, but the other ranks don't have number limits" would have been fine! >.<
Alright, so now's your chance to ask about anything I've said, any other questions that you've come up with, and then I want you to perform all the techniques you've learned thus far. This way, I'll be able to see how you think the technique works, how you would use it in battle, and I'll be able to see what techniques you still have to learn.