Powering MCU from both USB or Battery
The circuit looks good and shows a lot of thought went into its design. One thing I would add is a 1K resistor from the gate of Q1 to Vin, mounted as close to the gate as possible. This reduces noise pickup by the mosfet (the gate is very sensitive to HF noise) and protects it against voltage spikes. The gate is protected by built-in diodes but current limiting protects the diodes.