> it's pretty easy to test a multimeter for continuity. put it in resistance mode, it should show infinity or a very high reading.
put the probes together, it will read 0.

Thanks, I did this and it works as expected.

> would be good to verify the resistance goes to ~0 across the button when you press it

Can confirm it goes to ~0 when the button is pressed.

> you should also check the resistance between the switch's gnd point and another gnd to ensure that isn't broken.

Confirmed this as well. I went from the ground of the bootsel switch to the ground of another switch.