While the primary idea sounds like an interesting concept, some parties do not have enough support in all three regions to sustain a primary. Also, having delegates elected is a more complicated process and less engaging for party membership as opposed to the current process, where all members are delegates. In addition, I believe that it is best for the parties to have the freedom to choose when they want to hold their primaries (some may not want to hold them at the same time).
Finally, I believe, as a party leader, that it is best for the party itself to decide how to run their own conventions and the voting process, as the leader is best in tune to the system that the party is best suited for. For example, point 7:
The leader is the best person to decide who should participate in the party's primary, not the regions.