I usually insert the foot bed, then check to see if the heel of the orthotic is tignt against the back of the liner. Place my foot in the liner and verify that it is seated all the way. Then insert the liner into the boot. sometimes I then check the orthotic again to make sure it is set all the way back. If it isn't I will reach in and gently lift up the outside edge of the orthotic and wiggle it back into the heel pocket. You might want to verify that there isn't a lump or small fold in the liner that is pushing the orthotic forward. I know getting that liner into the boot is a bear and sometimes the orthotic gets pushed forward a little in the process. When you insert your liner, don't push down on the heel pocket while inserting the liner. If you can, put your hand inside the liner, palm up and fingers near the toe and "roll or press" the liner into the shell with the hand that is inside the liner. Pushing on the heelpocket of the liner to get it into the shell will also push the orthotic forward a little. (does this make sense?)
Yes the arches will cramp for a while as your feet get used to the orthotic, as you develop your technique more and learn to relax your feet, and if your orthotic was made correctly, you will find that you don't need to "clamp down" your buckles as tight to not move. This last trip as I found my center better, learned how to "let my body find it's balance" and worked on my edge angles I found that I almost don't have to buckle the 2 bottom buckles. Even in bumps and steeps I found that just enough pressure to keep the buckles snapped was enough. No movement of the forefoot. Try wiggling your toes up and down and flex your ankles a little while you ski. If you can't do this your boots are too tight over the forefoot or you are trying to grip your toes to the footbed (bad).
Sorry for the long post. Hope this helps.