That's what we're doing!
Fans have come up with excuses for the continuity glitches in STXI just as they did decades ago with TOS, TMP, TNG and the rest.
The difference in this case is that these glitches are too big and too numerous to just chalk up to somebody misspeaking. It's clear that something a lot more has happened than a disgruntled Romulan miner fell through a wormhole and started blowing shit up. Things are just too
different, even before the Narada fires a single shot.
And Spock, as we know him, has far too much experience with time travel, and its ramifications, to behave as we see him behave in this movie. Even if he was suffering from full blown Bendaii's Syndrome, he'd still be inclined to solve the problem by going back in time and preventing Nero's incursion in the first place, if that was, in fact, the true source of the timeline disruption; the only difference would be he'd need a computer to make the calculations instead of doing them in his head, like he was shown to do previously in TIY and TVH (and you want to talk memory problems?).
Occam's Razor applies here. Simplest solution is that it's not
our Spock. He came from another
parallel universe, where he never had those time travel adventures (and thus no experience in resolving problems like this), and either time-traveled back along his own timeline, or was shifted into yet another alternate universe.
Either way, the mainline Star Trek timeline never comes into play.
I say push the divergence point further back, to the Temporal Cold War, and blame the whole thing on Archer.