The Gorousei most likely allowed Spandam to go after the blueprints because they know he is the best man for the job. Imagine if one of the Gorousei themselves or one of the three admirals had gone, it would be something the whole world would know about but a small government agent that nobody has heard of going after them doesn't bring up too much hype.
Anyway, back on the topic of Robin's flashback, I believe without a doubt we will see, at least, one of the three admirals during this flashback. The Buster Call can only be issued by these three men, this CP9/Spandam business is the only time that someone else has been able to use it. I also believe that Aokiji gave Spandam the authority to issue a buster call is because he fears he will be beaten by Luffy, the next time they meet.
As a highly ranking government official, I'm sure he will be aware of the true details of the Alabasta events. How Luffy managed to learn of Crocodile's weakness and use it to defeat him. Or prehaps he has encountered another D, that has similiar traits to Luffy, with the ability to see a person's weakness and exploit it again later on.