how long til I can drive & function properly after expander to implant exchange

Bec, are you seeing a physio? I didn't enjoy expanders, but the implant change over was not as painful and I was driving and doing most things quite quickly.
I think you might need to push a bit harder to get this investigated, there are so many variables and everyone's body responds differently. Weeks of pain that restricts your life is a miserable experience, physio may help loosen your muscles a bit. Expander to implant may be viewed as the 'simpler' form of reconstruction, but it is still a significant operation. Mxx