I think PTS might be the best option for this pup. Either that or you take the problem off your BIL's hands now - tho he will probably go out and ruin another puppy. You could tell your story to the SBT club, and especially the breeder of that puppy. If the breeder was ANKC registered (read dog obsessed) they may well want their SBT back and be able to help you.
The worst problem is - what if it's some unsuspecting person that gets bitten by this dog next? A meter reader? A tradie? A kid trying to get their ball back from over the fence?
You could have another chat with the RSPCA and the Police about what criteria need to be met in order for them to step in and prevent disaster. I think the council would need the police to go with them anyway if they wanted to get the dog - given how much of a thickhead your BIL seems to be.
It would be very hard to sort a SBT that was fearful and nervous to start with and was then abused for its entire life. RSPCA would probably love to help but it's hard to define dog abuse well enough unless the dog has no water and no food all day - now. Especially the water. An "enriching environment" is hard to define, can be slightly different for every dog, and hard to measure and enforce.