The left side is for modifying the main set of flags (and some form flags), while the right side button is for forms only.
Normally, the complete dex only sets nonshiny flags because not all legendary flags are legal as shiny. Holding control will bypass this restriction, like in prior editors.
Editing Rowan and his aide or changing the number of mons needed to get the National Dex wouldn't be too hard I imagine. Most of the game's scripts are easily accessible in the Dpr folder and the scripting language is fairly similar to that used in Gen 4. The main problem is editing the actual dex itself so it actually shows any replaced or added Pokemon. I've looked almost everywhere in UABEA and can't seem to find it.
Back in the DS days all of this info was stored in one file and so it was a matter of just changing the order of the index numbers or adding new ones. But so far in BDSP I haven't been able to find the equivalent file, and despite being able to successfully change the dex numbers of individual mons I can't actually have the Pokedex screen itself reflect those changes.
This would obviously be important for a Platinum mod as you said. Right now I've even been able to mod the gym leaders and E4 to have their Platinum teams, but it would be nice if seeing these teams would contribute to dex completion.
Encryption constant is used to encrypt/decrypt a Pokémon file in-game, but it is also in effect just another identifying aspect of an individual Pokemon (which is perhaps partially why for BDSP, they decided to use a Pokémon's encryption constant as its identifying variable for Ball Capsule assignment).
Use the Block Data editor. Scroll down, find and open the Player property. The first property inside is BikeColor.
0 - blue
1 - red
2 - yellow
3 - green