I work in this field. Park area closures are all so different and can change by time of year, migration patterns, risk to the public, habitat, etc etc. so it is not reasonable to have signage everywhere (nor wanted, this is a National Park and the addition of anthropogenic “ stuff” on the landscape is meant to be limited). So I would not use this in your defence.

Did you use AllTrails to plan this trip? If you did that’s your first problem as it’s garbage and is just poor planning. Don’t use this as a defence.

That said reductions are common by being cooperative and showing them you understand where you went wrong and how next time you now know where to look up the appropriate information regarding closures etc. You can represent yourself or hire a lawyer .