Do I need different isolations among long-wire switches?
You should be fine having the shared power rail(case B), although I'm not sure how many switches you could be dealing with, so you might want to calculate the max current draw with all of the switches closed, which would be:

Current through one switch: (12-Diode_voltage)/680

Current through them all: Number_of_Switches * Current_Through_One_Switch

So long as the DC-DC converter can put out enough current (which is likely, unless you're going to have 100s of switches) it'll be fine.