Your first boil is completely unnecessary. Mashing at 150 takes care of most beasties and anything that tries to take root in there will be discouraged by heat (100-110F or 38-43C) and by covering your vessel and purging the headspace with CO2. Butyric Acid bacteria and Acetobacter are your main culprits of Berliner problems and both work to convert alcohol to the dreaded butyric acid and acetic acid (vinegar) respectively.
However, your initial fermentation is using Lactobacillus Delbrueckii to convert sugar to lactic acid. Lacto D. is a known homofermentative species and as such will not produce anything but lactic acid during its fermentation process. Your wort will contain sugar, starch, proteins, lactic acid and water, but no alcohol until you introduce yeast after your proposed second boil.
Furthermore, once the wort is inoculated with Lacto D., it will crowd out competitors like a yeast when introduced in a large enough quantity, like let's say, a smackpack. And once it takes root, it will work rather quickly to lower the pH below 4.5. Again, the low pH is in your favor and makes it hard for undesirables to gain any headway.
Like any other beer, with proper sanitation and a little preparation, you can sour mash easily and safely with very little to worry about.