Swap the red and blue stator wire between switchboxs, if problem follow its a bad stator, if not it a bad switchbox and replace in pairs only. No cheap Amazon junk.
Mercury outboards have not used a "mercury filled" switch for on engines above 100HP for around 25+years. If it has the red stator thats most likely the culprit as they where POS
You don't need gauges to test recirc valves or pressure fitting. Pull hose off recirc fitting while running, if fuel emitted from it is not a fine mist it needs either attempted cleaning or replaced. The pressure fitting for oil tank is simple, use it a day as normal then check tank the next...