As for the switched 5-volt supply on the Mayfly, it’s a boost converter that I put on the Mayfly years ago because we use a lot of sensors that require 5-volt excitation, but then they return a 0-2.5v analog signal, which is safe for the Mayfly to read with either the onboad 10-bit ADC, or the auxiliary 16-bit ADC. The 5v boost isn’t needed for working with the Grove rangefinder I linked above. I have successfully used them with the Mayfly before, and they are fine being powered by the 3.3v supply of the Mayfly, which is preferred since the signal levels between the Mayfly and any peripheral sensor should be the same. If you power an external device with 5v, then you should use additional level-shifting circuitry to protect the Mayfly from potential damage.
The VCC pin of the Grove ports on the Mayfly are powered by the SW_3v3 line by default. Swapping the jumper headers next to the jacks will change them to 5v, but only do that if you are prepared to deal with the potential voltage mismatch. To control the SW_5 and SW_3v3 pins of the Mayfly, simply write pin D22 high whenever you want to power anything connected to the Grove terminals.