Re: 1yr old getting out of harness!
First I would make sure straps really are tight, I have several friends who tell me that the straps leave redmarks but in reality they are so loose they are hanging off the shoulders.
If they really are tight enough maybe do a button up shirt on backwards over the harness. Just buckle her in then put the shirt on, I would probably even button any buttons I could in the back. That way she can't even get to the harness.
Is she maybe loosening the strap? My 6 year old cant even get her arms out of her harness without either unbuckling herself or loosening the harness.