I think Ellora's Cave did sell solely through their website for some time, and the move to start using other etailers is relatively recent (last few years?).
There's nothing to stop non US residents registering US copyright, as far as I know. There's an option on the website to select another country while registering online, so I can't see why it would let you get that far and stop you further down the line. There are advantages to registering US copyright, though probably not in a situation like this (there was a non-US author a while back who was plagiarised, but could only claim against the US defendant from the point where the book was registered in the US. Wish I could remember names! It was quite a big case, a romance author)
Of course, with Resplendence, they should be registering copyright, like most reputable publishers do.