I believe the best Optus can do is break the charge down over 3 bills
You'll receive the phone straight away, you don't need to wait for the old charges to be paid off first.
Yes you're signing a new contract so they'd complete a credit check again. If you're trying to get a new phone whilst owing something on your account you'll likely find it difficult.
Also what does it mean when I search up my order on the Optus ap it says under in progress 2 of 2 products of this order an it has underneath delivery to address an it has my address but I search up the other order an it only has under progress 2 of 2 of this order