At first it seems like the animators have forgotten about Dorulumon in episode 15, but then you realize that to avoid causing damage on the statues due to his size, he's most likely kept in Taiki's Xros Loader.
Taiki's second outfit has a jagged white line separating the red and blue portions of his shirt. In circuit design, zig-zags symbolize a component called a resistor - a fitting symbol for the man leading the resistance.
Similarly, an Omega appears on Taiki's Xros Loader when evolving Shoutmon, and a Z appears on Kiriha's Xros Loader when evolving MetalGreymon. Omega was possibly chosen as a nod to Omegamon, and Kiriha's Z is possibly a parallel to Taiki's omega. Alternatively, though, Omega is the symbol for an ohm, the physical unit of electrical resistance. Z is the variable most commonly used to represent impedence, which is the effective resistance of a given circuit component.
In most versions of episode 19 Deckerdramon tells DarkKnightmon he refuses to be a part of his team because he can sense no love in his heart. However, in the english dub Deckerdramon declines because he can't even sense DarkKnightmon's heart to begin with. Basically meaning DarkKnightmon has no actual soul to speak of.