USB and coin cell battery switching
Electronic industry already has a solid solution for this case. It is solved by a special class of devices called "power distribution switch", within PMIC group of IC. Search Digi-key for this type, "PMIC - Power Distribution Switches, Load Drivers", then select, say, "2:1" in the column "Ratio - Input:Output".

Essentially these are switches based on FETs, as "ideal diodes". Here is an example, FPF1320UCX.



A similar line of "PowerPath, Ideal Diodes &amp; Load Switches" is offered by Analog Devices, see something like LTC4413 



I am certain that many other manufacturers offer similar line of products. When comparing cost of solutions, you need to take into account not only the cost of individual components, but also the cost PCB area, and placing/soldering costs.