1 - Gray's Devil Slayer Magic only activates when he's facing (using magic against) a devil.
2 - He wasn't facing END.
3 - END isn't even a demon right now, it's only a book containing a demon inside (or similar), so DSM wouldn't be necessary to destroy it.
4 - I prove my last sentence by saying that Ignel told Natsu to not destroy the book of END.
5 - I only saw Natsu using Dragon Force at the heat of the moment, dunno if he can use it at will or not.
6 - If he can use it at will, maybe one of the reasons is that he doesn't has Ignel inside him now.
7 - Wendy also activated Dragon Force, don't forget her.
8 - I think that Gajeel also did on the underwater fight, don't remember quite well.
9 - Gray's power up will only be useful against devils.
10 - Regarding Memento Mori, well, Silver was a strong mage, and he passed his power to Gray, and Gray used DSM to absorve the attack, just like Natsu does with fire.
11 - So we can say that while Dragon Slayers absorve elements, Devil Slayers absorve curses (Memento Mori was one), probably... dunno.
12 - Probably all of them will lose their power up at the end of this arc, because of reasons (Gray - no more devils, Lucy - going to "resurrect" Aquarius, Natsu - ???)