I don't see how the beer can really continue to sour once the pH and alcohol get to a certain point. Brettanomyces is the one that will keep fermenting once the rest of the crew gets tired. If you are going to keg it, you won't get any more souring once you chill and carbonate it. Lacto just won't work in those conditions. I would wait a little bit to see if the krauesen falls and you get no more fermentation activity then chill and keg.
At certain ph and alcohol percentage bacteria will drop off but not all bacteria is as weak as the WL/WY lacto strains. Pedio is also used to sour some beers and will keep going right alongside brett. You don't get 10% ABV sour beers without it (e.g. Bon Chien). I know pedio isn't used in berliner weisse but even wild lacto can survive against higher ABV.
Lacto will sour at cooler temperatures but much slower than at ambient or typical beer fermentation temperatures. That's one of the ways milk goes bad in your fridge. Next time you let milk run too long in your fridge, give it a smell. You'll be surprised by how similar it smells to a berliner weisse. However, by the time the beer hits stable gravity there won't be much or any more sugars that lacto can consume (at least the WL and WY strains). Lacto can break down some complex sugars but generally not the ones left behind after sacc has eaten what it can.