There is a thread somewhere that goes into the clutch actuator being jammed up from old grease. This makes sense here, because a new clutch disc will inot help the shift in reverse problem if your clutch can't engage and dis- engage. I have this same problem of an intermittent failure to shift into R after my smart sits for too long. After I fiddle with the shift, eventually it operates fine, and as long as the car gets used, it doesn't fail to shift.
I have a 2009 Brabus. It's had that update, of the TCM AT THE DEALER IN 2010 I think.
Since then I have had DP TUNE re flash my ECU, AND TCM. No problems with that, and now after a couple years this failure to shift came around. So, at my 40 K service, after reading those old threads, when I went into MB smart Houston north, I told them about my problem, and they said this is a common problem, and re flashing the TCM FIXES IT. Well, I hired the dealership to just take my actuator apart and just grease it, and NOT TO REFLASH my TCM.
They did this and I will let everyone know how it worked. I have a strong opinion it will help, just because of that thread about a crusty actuator, and how it works.
Maybe someone can add the link to this thread here.
Hope this helps, MB Houston was all about that TCM update. I love the DP TUNE update, and it didn't cause any problems before, and the weak link has to be putting the clutch IN before the shift, and it wouldn't shift in gear on a manual tranny car, if you didn't depress the clutch pedal, so why would a smart shift into gear if the clutch WAS engaged??
Get your actuator disassembled and greased, like I did, and see if that helps. This is preventive maintenance and they charged me $149 for this service. If you don't have yhe update,get it done, AND have them grease the clutch actuator as well.