In terms of strategy against Navy, the 4-2-5 should be decently effective. However I would play Cash and Cockrell at safety at the same time. Run support from the safety is extremely important, especially to a team whose LBs aren't the surest tacklers. I am not as worried about Navy airing out like GaTech did against us and feel more comfortable with our backup corners on islands against their wideouts.
Bring Cash and Cockrell closer to the LOS in the "hybrid-LB" role, along with Kelby and Helton, to something more akin to a 4-4-3. Or sub Cockrell from AYW/Norman who are better in run support than coverage, and drop Cockrell as the lone coverage safety with a combination of Patterson/Fields/Borders rotating in and out for the two CB spots. Cut has said that Russ can play any of the secondary positions well, why not try it out?