You'll have to forgive me if this is rambling, because I'm trying to write this down to make sense of it all. When Mother died last night, did she not say "Thank You" to MIB? It seems to me she conned him into killing her so she could finally die. Perhaps she was immortal from drinking the wine and couldn't kill herself...and perhaps for some reason couldn't be killed by the Older Others--much the same way the candidates can't die unless they kill each other?
We know they weren't originally immortal because both children aged. It wasn't until MIB became Smokey and Jacob drank the wine that their appearances never changed. So did she long con her "special" son to kill her and imprison Jacob by giving him the immortal sip of wine?
If this is the case, that could be the reason that Jacob didn't seem to mind being killed by Ben--shocked, yes--but egging him on in the process. So while MIB thinks he found the loophole, it seems it's precisely what Jacob wanted all along, the same as Mother.