While I'm on the train using my AirPods with my Mac, how do I accept a call from my iPhone over the AirPods while using my iPhone as a Wi-Fi hotspot?
This isn't currently possible as AirPods connect to one device at a time and your Mac isn't able to accept calls unless it is on the same Wi-Fi network as your phone (Hotspot doesn't count).
Until one of those changes you'll have to ask the caller to "give me a minute to put on my headphones" (common practice) while you open Bluetooth settings on your iPhone (fastest: swipe up control center and choose AirPods in AirPlay) and connect to your AirPods. The call will automatically route to your AirPods once connected!
Alternatively you could get a bluetooth device such as the Bose QuietComfort 35 which allows you to connect up to two devices so you could be connected to both your iPhone and Mac and switch easily between them!