Here's how I've been doing them.
Mash, sparge, then bring to 180F+. Let it cool down to 120F or under then pitched L. Brevis or your preferred lacto. Taste it or check pH after 2 days, it's going to be tricky and personal preference after this. The wort will still be sweet but also sour. Once it reaches the sourness that you want you can either boil it to kill the lacto or you can pitch yeast or brett or both. I've stopped boiling it to kill it recently and I like the results. I've stopped using hops in berliners as well. You're just working against yourself. You can try dropping them, works really well.
As far as keeping your heat up you can use your mash tun if you use a cooler, heating pad, brew belt, space heater in a small enclosure, aquarium heater, etc. I've been pitching L. Brevis at 120 and letting free fall and it sours pretty well. You could also try using Omega's Lacto blend which works well with a free fall fermentation profile supposedly.
I have never used the lacto from grains because, to me, it's sketchy and unreliable.
