[RIP]

Sometimes additives can create more problems then they fix. Only use them as a last resort.

First thing I would try for your oil leak is change the PCV valve. Why? If it's stuck closed crankcase fumes and pressures build up to the point it releaves itself at the weakest point which would be a seal. They are cheap. Change it.
Next, sounds like the rear valve cover seal is shot since it's right above the rear exhaust manifold. To confirm it I would clean the oil off then put a dye in the oil and trace the leak with a black light.

The horn? Check for 12 volts DC at the horns under the hood. Zero volts? Probably have a bad horn switch in the steering column. 12 volts? You need new horns.

I believe your engine uses a timing belt. At 79K miles you need to start thinking about getting it changed. If it breaks you may need to buy a new engine.