What I understood from the storyline is that Grima follows Lucina to ensure that he gets ressurected in the past. He tries to get control of Present!Robin, just like Future!Robin, but fails, causing the amnesia, and giving him bits of the Future!Robin memories.
Of course, it ends up causing a paradox in which
two Robins exists, enabling Grima's death by the hands of Present!Robin, because the game says he's Grima, too.
Something, I must say, still confuses me. I see Present!Robin as Grima's
vessel, which means he's not Grima yet in any way. He has (or had) the potential to become a god of destruction, but that's about it.
But eh, let's go with it: Technically, the only way Grima would be able to revive would be via assuming control of Robin, his vessel, therefore, assuming control of the only thing that could kill him for good.
I still don't understand exactly why Grima didn't ressurect himself earlier, honestly, because
it's shown that he could have done it alone with Bad Future!Robin's body. Or why he didn't try to assume control of Present!Robin again, before his bonds with Chrom and the Shepards got too strong.
The idea that amnesia helped him deny Grima is because when he mets Chrom he can't remember anything about his past, and ends up becoming more attached to the present, than to it, when he was with the Grimleal. That means his past wouldn't be a hindrance anymore to his relationship with the shepards, as it probably was with the Bad Future!Robin.
I can understand why he would still try to get control of Robin at the end, seeing as Robin was, for some reason, the only one able to kill him. But really, he already had one Robin under his control to do his evilz stuff, so why couldn't he just kill Present!Robin when it was easy to do do?
The Bad Future dlc give us base to the theory that the Bad future timeline stayed in another dimension when Lucina and cia travelled to the past, so killing Present!Robin shouldn't change anything for Grima.
That theory shows us that Grima travelling to the past after Lucina was useless, too, and even got all his versions killed for nothing, because he could just stay in that other dimension doing all destruction he wanted.
The storyline has some pretty big holes in it's plot, and yeah, it's nothing too Earthbreaking, but I still like how it executed some cliches pretty well, such as "the amnesiac" and "FRIENDSHIP POWAR is stronger than EVERYTHING" tropes.
However, the best part of the game lies within it's characters(via support). That's why I say it's the game's strongest points: The difficult is nothing to look at, the storyline, aside some minor points, is pretty bleh. The characters, however, just painted this games the color it has.
I mean c'mon.
Still, it didn't make the Greil Mercenaries' appearance any less awesome! 