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OK,
Just to make sure. Remove the switch from the bottom of the tank, it has one screw the retains the switch in place. Disconnect the two wires. Connect your ohm meter to the blue switch wires. There should be no continuity, the meter if analog the needle will not move, if digital the display should show OL indicating the circuit is open which is good. If the needle moves or the digital display shows 0.000 or some resistance then the switch is bad. When you said you disconnected one of the wires to the switch and the horn stopped, you didn't mention if the tank was in normal position or upside down or I missed something. If the switch tests bad, it's only $20 to replace it. As far as the tank is concerned, if you hear the float moving up and down when you flip the tank over that's good and indicates that it is not stuck. I'm assuming you have disconnected the oil line from the bottom of the tank and drained the oil out of it? If so, this is the time to replace that oil line from the tank to the oil pump, you must use a replacement oil line from Mercury and not just regular clear vinyl hose. It comes in 15" lengths which will be enough to replace the hose and may need to be trimmed to the correct length. There is a procedure to restart the engine after you have replaced the oil line and refill the tank. DO NOT restart the engine without doing the procedure that I can walk you through once you have it back together.