Swamped

This might play to your advantage honestly, if the Bogknights are victorious in repelling the battle, than with all the confusion of an full on attack, nobody will piece together the various clues that you were behind mudviper's breakout, they will just think you were captured during the heat of battle. And if they fail... well it doesn't really matter then does it, nobody but mudviper and whoever set you up to break her out will know. It might weigh heavy on your conscience though, especially considering the incapacitated Nathan, who wont be properly warning the base of the battle.

Even if the full marshguard regiment cant be recalled, maybe you can still get to Rider and get a partial recall of the marshguard forces they are in direct contact with. Of course even though it's an immoral fight and gross leap in aggression, now that the orders have been given and there is no way to properly communicate a retreat to the rest of the troops, there's no advantage for the rider and the marshguards to not put their full force into the fight. You are going to have to convince rider somehow to get the troops directly under their control not to participate, which might be tricky.

The only answer I can see is to have Mudviper leverage yourself as collateral to convince Rider to let a loosing battle take place. You are worth something to the marshguards, maybe enough to shift the tide of this battle. Appeal to Mudviper's emotions to get her on board with this, she knows the bogknights aren't bad people, just a diametrically opposed force. She must know someone in the base too because of the whole note thing, just cross your fingers it's someone worth going up against her commander for.

If she leaves you here, hidden, she can come back for you if she's negotiated a losing battle (or at least a weakened marshguard force) and if she doesnt come back for you, then you return to the base as a traitor and hope you can do something to help with the battle.