Regarding the elimination of the DCS/DCC selection switch I'd bet it wasn't a cost-reduction move. It was probably a customer complaint reduction move. I wonder how many people, unaware of DCC at all, accidentally flipped that switch and then called MTH or their LHS complaining about this hunk-of-junk that suddenly stopped running.
Documentation about the missing switch from MTH does in fact say it's due to numerous calls from confused customers. I suspect it's due to the typical buyers of Rail King vs Premier locomotives. Rail King are generally not model railroaders, so they'd get mixed up with that switch.
Sad to hear about your issue. I've opened and DCCed two of the RK -8's myself, but neither where Christmas ones. Those middle tank screws got me both time, forgetting that they also hold the body on. The outside end screws were easy to figure out they held the body on. But getting them back together was a hassle both times, amazing how much they shove in these bodies.