I don't remember if there is a way to lower corruption. It could be hard. This is why it's important to think about making choices that could make it higher. If you play with an activated dev mode, then you can see all the changes in corruption points that occur and rollback your choices if necessary.1. Unfortunate, oh well if it will eventually happen then that's good.
2&4. Ok thats good but is there a way to lower corruption for Alexia or Rowan then if I take this route? For that matter is there a mechanical advantage for Rowan and Alexia to remain pure/become corrupted? I briefly ran a save where I used the developers console to max out rowans corruption and nothing really changed I think.
5&6. Ok will check that out.
3. Can I ask how that route ends because from what I can tell there don't appear to be any scenes for it in the scene replay mod gallery? You can spoiler tag it to hide it for other people but I am kind of curious what happens. Or can you tell me how to trigger the event using the console? I am pretty sure I passed like 15 weeks after the event where Rowan challenged her and nothing has happened.
Corruption doesn't give Rowany any bonuses to stats like combat, diplomacy etc., but is important for the plot. For example, Rowan's reaction to what happens in Rastedel differs significantly based on his corruption level. For Alexia, a certain level of corruption is necessary to trigger some events.
Cliohna's route has two endings, depending on your choices. In the first one, Rowan and Cliohna establish fairy even relations of common interest. In the second one, Cliohna gains magical/sexual control over Rowan. The Act II is supposed to built upon the model of relations between them that was chosen in Act I by the player. I don't remember specific names of the Cliohna events. They appear quite early in the game and I never needed to use console for them.