don't make a starter, you'll upset the balance of bugs (lacto doesn't like oxygen, while brett loves it).
12% is typically listed as brett's alcohol tolerance so you're throwing it into a rough enviro. i would consider getting another wyeast pack of just brett and adding that too, to up your brett cell count and thus improving your chances of finding some hardy cells that can stand 12%. a roesalare pack doesn't have that much brett in it, i believe it's something like a quarter the number of cells you get in a brett-only pack. don't get white labs, the cell counts in their brett tubes is much lower than wyeast.
brett supposedly does feast on autolyzed sacch so maybe it will clean things up... but i wonder the brett eats up the dead sacch right away, before it has time to really decompose and create off-flavors - so in this case, since the sacch has been dead for a while the off-flavors might already have been created. in other words, even if the brett eats the long-dead sacch the off-flavors, once created, will stick around. i don't know if this is the case, just wondering out loud. please do this, give it a year, then come back and let us know!