It might be very difficult to strech. It might just fall apart when you first mix it up. It might come out perfect (I wouldn't overwork it). I don't think you'll need any more starter. If anything, I'd use less preferment - say 25% of your total formula as opposed to 31%. If it falls apart due to too much acid and enzymes from the preferment, no amount of working it will develop the gluten and save it.
If it was me, I'd make the back-up batch. There is a good bit of uncertainty around your new formulation. Though, if the dough comes together and feels strong, and you can get it smooth, after mixing, I think you have a very good chance of having a good final dough. If it's going to fail, I think it will do it when you mix it all up - not when it is proofing.