Seriously, why could he not have done this? Is there some aspect of Time Error associated with the DBT that prevents changes to it once they've been made? Or is traveling to the DBT an act of time travel and thus protected by TTI (meaning Schala will be TB'ed regardless of if the above happens)?
I'd say it's pretty much that. If he could just save her from 12000 BC, he'd have just done that. I doubt he wouldn't considering his priority was always Schala. However, since Time Error wise 2300 AD was still the bad future when Schala got sent to the DBT, then that option was out since he could do nothing about it at the time.
So now, he has to resort to this complex-plan. So yeah...