Don't use alcohol or furniture cleaner, it is bad for guitar.
A good idea is to keep a dry rag in your case at all times, and everytime after you play just wipe down your guitar. Or use a dilutant of water and rubbing alcohol, then let it dry for about 10 minutes. Strings sound brand new after that.
If you own a cheap guitar, you can use furniture cleaner on it. If you own a nice guitar, anything over about $200, then it would be a good idea to use guitar polish, from a music store, on it to clean it up. You don't want to harm the finish on a nice instrument.
NO, a guitar neck needs special products so you don't dry out the wood of the guitar. If you want to clean it rub the back of the neck and the fretboard LIGHTLY with 000 grade Steel Wool. After all the grime is cleared off, you can use lemon oil based products wipe the neck.
I wouldn't. Most instruments have a varnish on them to make them look glossy. Furniture cleaners can be too harsh and strip this varnish away, or discolor it.
No because funiture cleaner has wax in it and if it gets on the frets of the neck or on the strings you will be replacing guitar parts and that gets expensive.