I voted option 2 but I do like the 48 hours of containment before coming out of PG idea. I've read a number of PG threads on promising cows where the OP puts their effort in, and there's some constructive criticism, OP updates the post to include more information and/or make the OP more readable ... and then it just stays in PG, never going anywhere.
People like Adrian Harrop, Saira Rao, smug_legend, all with strong thread potential, the OPs went through the improvement process, but it didn't result in getting the thread moved out of PG. And they might not have been mega-threads, but there's lots of room in the site for smaller, niche lolcows.
So to me, Proving Grounds is a good idea in theory, giving a thread the chance to get more relevant information in before a potential DFE and make sure it's not automatically terrible. But I'd lower the requirements for a thread to graduate onto the main boards drastically - and if it turns out the cow doesn't sustain the attention, then the thread just fades away unless they do something that gets attention, like shooting two cops.