Mr. Johnson:
Now... how do you make it work? What node or nodes do you need to take over in the system that will make it vulnerable to the flood of legitimate outside requests where you will hide the forged data?
Well, he's got a 'backdoor' through the agent terminal, right now. His earlier research probably told him where the "logjams" are. [As a wild guess, the two bottlenecks are delay/timeouts in form entry and various printers taking too long- IDs, that sort of thing]
So, what he needs to do first is adjust the printers to work more efficiently (He can use an old "ink-saver plan" idea, and use microscopic "checkerboard" patterns in fonts to allow the system to print faster and cheaper. That's as easy as finding and modifying the node responsible for Physical Printing, and then dropping slightly different code in it.
After that, use the system terminal to enter the single most plausible of the 'chaff' the Johnson gave them. As a test. See where it hits a log-jam in the system, then center on those areas.
Probable plan of 'attack' after that would be to put a timer on the various bits of programming that apply to the trouble spots, so it looks like a single, benign Update rather than an individual working on a bit here and a bit there.