ARP head bolts on an LS6: do they eliminate the GM stretch procedure, and can heads come off with studs in the car?

Heads off with studs, in the car: often not. Speedway notes studs require the heads to be lifted straight up over the full length of the studs, which can make it hard or impossible to remove the heads with the engine installed. That's a real consideration versus bolts.

The torque procedure: the factory GM bolts are torque-to-yield and single-use. Aftermarket bolts and studs are reusable after inspection. For the exact torque procedure with a specific aftermarket bolt, follow that manufacturer's instructions — we don't have ARP's LS procedure as a source.