Wouldn't make much sense given how Richard seems aware of who Jacob is in 1954.
Could still be possible I suppose but that would involve more cheap time travel bits. And still woulldn't make much sense. Meh, I really wish they hadn't decided to use time travel to anchor the plot like this. It smacks of desperation.
Well, a lot of people tend to think that Jacob is someone we've already seen, and have thought that since late in Season Three. Maybe we're wrong, but if we're not, that means that some sort of time screwiness had to occur before we even knew what was going to happen this season. Granted, I think that people that aren't fans of this show resorting to blatant, physical displacement of characters in Time might be more willing to give the spirit of a deceased character a pass, as they are no longer "enslaved by Time and Space" (hear: the Room 23 video played backwards). So maybe we won't learn Jacob's identity until a character dies (or if it's Locke, that's already happened), as Ben or someone else can't trap a living person.
EDIT: Also, we've already seen the time-skippers affecting the ancient past, and Christian was wandering around in a subterranean cave at that point.
More generally speaking, time travel is always a risky plot device and writers tend to rely on it far too much as an 'easy out' for any corners they've painted themselves into (which usually leads to inconsistencies and paradoxes that they didn't catch...or didn't care enough about to work through). With this show, for me, the verdict's still out. The time jumps this season were far too convenient to be mere chance. If/when we learn of the intelligence directing them, then I'll be far more understanding of such plot developments. If they decide to leave that up to fan debate, then I'll assume that they're just using that 'stoking the forum fires' mentality as an excuse for laziness. Given that The Island or whoever (or whatever) seemingly handpicked those it 'Raptured' from the plane back into 1977, I don't think it's just extremely convenient randomness.
This show has always had the Science v. Faith issue buzzing around between the foreground and background. I think in hindsight, it's pretty clear that what timespace is and how limited our perception of it is, is the other central issue of the show, one that goes hand-in-hand with the aforementioned debate. We're finally seeing a more direct application of the latter in the story, even if the storytelling itself is arguably more uneven this season.