High-low, or dual tone, horns are fairly common and the idea is to be more audible as a good portion of the population has hearing deficiencies toward either end of the audio spectrum. Even older American cars have this feature, it just sounds different.

When you turn on your defroster the AC engages as it provides drier air which aids in removing the condensate quickly, and also helps equalize the temperature on both sides of the glass which is the cause of "fogging". As for load on the engine, the VX has a rotary AC compressor which places minimal drag on the engine compared to the older piston compressors most commonly known as "York compressors".

John's explanation of the climate control is spot-on and I believe is outlined in the owner's manual as well.