- Shoichi accidently uses a 10 year bazooka shell and sends himself to the future and runs into Byakuran.
- Shoichi doesn't like the future he sees, and tries to change it. This sends him to a different future (a parallel world) where he is a musician and runs into Byakuran again. This causes Byakuran's power to "Awaken".
- Byakuran uses Shoichi to cause a past version of himself to "awaken", thusly giving him more time to carry out his plans quicker.
- A future version of Shoichi sets up a trap to erase a past version of Shoichi's memory, thus making him the ideal spy for Byakuran.
- Future version of Shoichi sends notes back to the past with instructions for his past self to do things, like: a) send Tsuna's Family to the future, b) Attend the same college as Byakuran, etc. To set him up in position to be able to being Byakuran down from inside his organization.
- Future version of Shoichi does this not only to set up the best possible scenario for defeating Byakuran, but because this is the only version of the future that has the power to defeat Byakuran since it is the only version of the future where Tsuna and Shoichi meet accidently... Thus establishing a relationship between the two that leads to the development of the Vongola boxes, and a level of trust with which to base future collaboration in the fight against Byakuran.
- Based on that trust, Future Tsuna uses Shoichi, Hibari and possibly Uni to bring past Tsuna and his family to the future to not only bring the Vongola Rings to the future, but to bring the Tsuna with the fatest growth, most potential and unpredictabiliy* to the future. Therefore he helps Shoichi in coming up with the plan and carrying out Step 5.
- To add to the possibility of success, Future Tsuna fakes his own death. This causes two things: 1. It stops the Millefiore from actively hunting Tsuna in the short term, allowing his past self to grow accustom to the future more gradually. 2. It immediately instills a sense of urgency and danger in his past self that he would not otherwise have had, immediately putting his guard up.
Did that make any sense?
*I think the reason why the Past Tsuna is potentially stronger that the future Tsuna is because he has more to lose, and therefore he will fight more desperately to protect what he has.