This is correct, and I think you have found an oversight on my part. My DM42 is bricked at the moment, so I was relying on my memory which was that the normal operation of [<-] was to remove one digit at a time from X until empty, not erase all of X at once.
I was also assuming that it would be obvious when X was empty, but as you say, it won't be entirely void - it will contain a zero.
My thinking was that if it could be tested for in code when X was empty, the next single press of [<-] could invoke DROP. Without checking the code, I now assume that there is no test which says that X is empty.
Re. terminology, I think it is a good idea to define some terms, and in this project, this privilege is of course yours.
Just for the sake of understanding you correctly, your fast double click and the double press concepts seem identical, except for the time threshold which discriminates one double click or press, from two consecutive, single presses. Correct?
PS: Don't change anything yet. I suspect that this might be a fart...