When I updraded my 622 to a 722K and added a 612, I went through the same dialog. I wanted to do my own install because the coax wirng in the my trailer is quite different from a house.
I have switches so I can use the automatic roof dish and a ground tripod. I also have a switch for OTA or TV2 routing to the bedroom. Also the compartment where the 722K would go is nightmare to cable because it is built into the wall. I knew most installers are not up to dealing with such things.
After getting to the a very high level, I conceded because they were not going to let me have the 612 otherwise.
Cost was not an issue as I am on the Protection Plan. When the installer came, I had the 622 out and I offered to put the 722K in and the installed could put the 612 into the bedroom. I think the installer was happy after looking at the space the 722K had to go into. The installer put the 612 in and tested both units and left.
Rules like this seem very irrational unless you look at it from the other side. A self install of an additional unit could damage the unit and Dish would be on the hook for it. There is no way for Dish to know how talented you are.
So let them install it.