ORing, cascading and reverse polarity protection for power inputs
Just add one diode (any you have handy) on VIN, before the THRU cronnection. Then raise VIN by .7v (or use schotky and raise .3v). If you can't raise VIN, you can try to lower VUSB a bit, so that VIN keeps having priority; just check that output is working despite having 0.1-0.3v less than nominal voltage. From what I can see, the IC works down to 4v, so you have room to play with...