I had the same experience when going through the Montana LLC route, Everything can be done by mail however when I brought the MV20 vin verification form to the CHP they told me they can't touch any out of state forms.
I ended up driving to Montana and making a nice road trip out of if it. Once I was there I called the Montana Sherriff's department and they dispatched an officer out to meet me. He was there in 20 min, filled out the MV20 which I then brought to the registered agent along with the rest of my paperwork .....within a few hours they emailed me temporary plates. A few weeks later I got the permanent Montana plates in the mail.
Montana was such a pleasure to deal with compared to California highly recommend it!