Swamped

Wait for the shift change, and sneak in. Pretend you are a guard up to Mudviper's cell, maybe you should get a plate of food or something so you look like you have some business to be there... you could even sneak in a key, or something sharp for mudviper to break out with. If you left a note of where you'll be later for her to "kidnap" you, it wont look as if you organized the jailbreak... of course it runs the risk of Mudviper letting the other Marshguards go, but you may want to return to the Bogknight base at some point, and this way you leave room to spin a good story about what you were doing.