When grim was smirking at the girl that seemed really angry from her life, rattling on about not living anymore on the beach, had he known that he would be intertwined in bedsheets shortly with the mortal? Most certainly not. If he had shown a glimpse of his coming days, would he put a stop to prevent things that should have never happened from happening? By all means, no. Writing the name of Ellen in the Death Note timelier or postponing it for whatsoever purpose had not a thing to do with the demon escaping the hell. If not Ellen, grim would postulate another plan to trap the demon.