I've had a realization about Spirit Evolution, in terms of what is it. Basically, it's an overlay. According to the definition of overlay that I found regarding data, it is one of two or more pieces of data that can be loaded to a predetermined memory region on demand. Additionally: it can be stored like any other piece of data, it can be changed with another overlay at any time and only one overlay can occupy that space at a time. Sounds familiar, right? - Drakohahn
When Takuya and Koji gained the ability for Unity Spirit Evolution, it always bugged me that the Ice Spirit was a key part for the Fire Spirit. However it just hit me not that long ago; the Ice Spirit for the Fire Spirit and the Darkness Spirit for the Light Spirit were for control. All the other spirits were used to up the power of the Light and Fire Spirits, but they needed a counter productive spirit in order to have that power be controllable.
If I had to guess I'd say it's most likely because Takuya and Koji had stronger or more aggressive personalities than Tommy and Koichi. It also might be because they were the ones who initiated the evolution.
Having the human become Digimon themselves may be an odd plot element, but it actually makes perfect sense! Look to Windward... I mean the previous seasons for comfirmation.
Season 1: The movie concluding the season ends with two Digimon physically merging together to deliver a thorough asskicking.
Season 3: A slightly different variation of Fusion Dance, between a physical human and a physical Digimon becomes the gimmick for that season.
Come Season 4, and we have humans physically merging together with the souls of various Digimon to fight as one.
Lucemon actually thought he was God. In The Bible, God is described both as a being of blindinglight and deepshadow. Lucemon Chaos mode had both of these attributes, except he was evil and crazy. He convinced himself that he was God and thus only he should rule over everything.
Although much maligned, the Royal Knights arc actually shows what is probably the most realistic portrayal of Digimon gaining experience and strength in the entire series. They go from being new to the form and getting curb-stomped, to giving the knights some trouble, to fighting them on even footing, to finally beating them.
How was Takuya able to predict what that storm would do after he returned to the Digital World? It makes some sense when you consider that the Warriors' combined form is Susanoomon. Susanoo was the god of storms in Japanese Mythology.
Why are the Human-spirit Digimon Adult-classed? Because, it a way, they evolved from the chosen children.