The method of using the test pads under the battery is detailed in this thread (look at Wonkos post for Data+,Data-.5V and GND for temporary USB connection). I flashed my secondary phone with it, so it is valid fix. Fortunately I could resolder the charging pins for the new usb port to pcb, so I can use the port for charging as long as I short circuit the data leads coming under the battery and if I need to reflash I just use all the four wires coming from under the battery.

Be aware that this method bypasses some of the protections that the normal usb-port has.