Bueno: If you download the mod available on the first page, you won't need a walkthrough.
The mod display which choices will increase or decrease your corruption points.
All you need to do to explore all routes is to maximize your corruption points (top right corner) and manage your true feeling point (top left corner) as follow:
1. Purity: Keep your TFP to the max. Always choose to "Stay loyal to Brian" when asked.
2. Corruption: Decrease your TFP each time you can. Always choose to "Degrade your relationship".
3. Blackmail: Decrease your TFP each time you can until they're equal to 6. Then, next time you'll be asked to choose between "Stay loyal" or "Degrade relationship", choose to "Stay loyal". Your TFP will be back to 7 and you'll be on the blackmail route.