Shev isn't especially well suited to take over Sin's "watch" which is happening off sight and which requires a bit of deception to pull off. The Umbaran in Imperial areas doesn't attract the kind of attention that a Mandalorian Warrior would although she may appreciate the thought. A local sentry would very much appreciate the added rest; would you want the circuit on level 3 or the more meandering patrol of of the lower levels. I'll note that a roof guard is not a standard post in part because it's outside and because it's exposed to outside observation which betrays activity.
So the "extra bike" that was going to get loaded into the trucks will be loaded. Sounds like the trucks will be fully loaded.