How do I block power from going to usb without stopping data transfer?
There is no easy way to do this. You could try to rewire a USB cable with the power wire removed. There's a chance this would work, but I wouldn't bet on it, it would depend on how the iPad handles USB connections. 

You won't ruin your battery by having it plugged in at 100%. This might be an issue if you're leaving it 24 hours a day, but Apple is smart enough not to let their batteries die from being plugged in.