1 - I've had decent success by starting my own lactobacillus cultures using wort with a small handful of grain, then adding the decanted culture to fresh wort. If you have time, it may be worth getting a couple of sterilized mason jars, adding a cup of 1.025 (lower gravity) wort and tossing a different grain in to each jar. This is what I did to get a decent sampling to figure out which one was the best. Each one was totally different, from the pellicle to the aroma and flavor. I then picked the "best" one and added to a BW.
I've been meaning to post photos and a write up of my "experiment". They'd be good photos for the "Post Your Infection" thread.
2 - I'd probably go off of flavor or pH, rather than strictly a time frame. When I do a sour mash, I've found that around 3.3 - 3.4 is the sweet spot for my taste.
3 - I've always gone higher with any BW that I do, but that's just because it's my personal opinion that if I'm going to drink a beer, I want to have at least 4.5%. If it's lower than that, I'd rather opt for a homemade rootbeer... but that's probably just my inner alcoholic talking. Higher concentrations of alcohol can give lacto some trouble - not sure of the exact %, but this shouldn't come in to play at all with the way you're doing it, since alcohol won't be created until after you've killed off the lacto with the 2nd boil.
4 - Lacto doesn't like oxygen, so purging the headspace with co2 is a good idea. When I sour mash, I always put a layer of saran wrap directly over the mash, then purge, then put another layer of saran wrap on. It really, really helps keep down on the vomit/death smell from the mash. I'd probably go easy with transferring the wort in to the fermenter - no excessive oxygenation, etc.
One final note, and the reason that I don't like doing it this way - either use glass, or a bucket that you don't want to clean ferment in again. I've found that once lacto gets in to any of my fermenters, it's really almost impossible to kill off.
If I misspoke about any of this, I'm sure someone will correct me.