USB, LiPo and External power
Looks OKish.
Note Andy's FET circuit for about zero fwd voltage drop.

Chosen LDO has 200 mV max dropout at 100 mA load and 25 uA quiescent.
With a LiPo cell and 3V3 output you want as little voltage drop as possible and 200 mV is significant. If you use a Schottky it's even worse -
3V3 + 200 mV LDO + 300 mV say Schottky = 3V8 !!!
Your LiPo will have much capacity remaining when the regulator drops out.

Even with the FET switch you have 3V3 + 200 mV = 3V5 minimum battery voltage.
The MCP1802 datasheet fig 2-13 suggests typical dropout of about 1.66 mV/mA at 25C - worse if hotter.  



An LDO with lower dropout may be in order if you want to use more of your battery capacity. and something like the MOSFET switch is going to be essential.