Rick, you have that exactly right. One wrinkle, though, is that the defendant has to ask first. (I know that's not how it looks on TV. Prosecutors can "mention" cooperating, but Defendants have to make the first offer. Cops, however, can promise all they want, since they can't agree to anything.) Once the issue is on the table, negotiations ensue. You can discern that Kabuki in the Manafort stories. Once the offer has been made, though, your sequence is exactly right.
The key for Manafort is identifying activities and explaining how evidence fits together - even if he never testifies. Gates was not "necessary" for Manafort's trial, but he could provide the view from the inside, so they used him.