I'm Japanese, but don't live in Japan. Still, I am going to try and answer your questions honestly based on my opinions.
1) For business, most Japanese will shake hands with a westerner (though handshakes in Japan are not as firm, and firm handshakes can make some Japanese 'tense up'). I would say head nods are better, or a moderate bow is ok, but don't attempt actual bowing until you have been there for a while and know how to do it properly
2) This depends. These days, there are many foreigners in Japan who speak Japanese, some very well. Some people are used to this and you will have no problems. Others will be so uncomfortable, as in not used to it, that no matter how well you speak, a look of puzzlement will be on their face. In urban areas this may happen less, though expect some people to respond in English, or tell you how good your Japanese is, even if you only speak a few words. You will probably experience a few people who will just run away while waiving their hands, maybe if you ask directions on the street. Don't let this get to you. These are all the possibilities you may face
3) Hmmm. Friendly isn't really a Japanese concept. Most people are polite and considerate, but there is a difference. I think the vast majority of people you will meet will want you to think favorably of Japan, though
4) In many ways, Japanese is easy to learn, BASIC Japanese, that is. Most foreigners are taught a polite, standard kind of Japanese. It is not informal, nor is it keigo, which is very polite. If you want to stay for a while, you will then learn informal and more formal ways of speaking. So, I think it will be easy for you to learn basic Japanese, but the rest may be hard. Please note- informal Japanese, like the way 2 life long friends would speak to each other, should not be used with strangers, EVER. There are also big differences between the way men and women speak. Anyway, I'm probably scaring you at this point ;-) Long story short, if you have a good ear and a good teacher(s), it should be ok for BASIC Japanese
5) Dating rules? I don't know if you are male or femlae. I guess it's different, but you are foreign. This may attract some people to you, it will also make others nervous. You just have to accept that
Finally, there are some things I would like to add that you can study further yourself, whether online or in Japanese culture books.
A) In Japan, there is a concept of 'tatemae' and 'honne'. Many non Japanese have problems with this. Tatemae refers to being 'diplomatic', and maintaining the harmony of the group, which is the core of Japanese culture. It could mean that if you ask a Japanese person you don't know well a question about Japan, you may get an answer they want you to hear, rather than a true opinion. This may be frustrating for you, but it has to be accepted
'Honne' are real opinions, or true feelings. Once you make friends with a person, over time, they will express their 'honne', but don't expect this at first. In the beginning, take questions you have asked people about J culture with a grain of salt, as they will most likely be 'tatemae'
B) Individuality is not fundamentally imortant in Japan, unlike in the west. Among Japanese, the group consensus always comes first- ALWAYS. We have an exression 'the nail that sticks up gets hammered down', which is cliche, but true. However, as a foreigner, this doesn't really concern you directly in many ways. You will see it in action, though, if you are observant
C) Finally, it is common habit in Japan to nod and say 'yes', 'hai', 'eh', or 'un',(don't say 'un' since it is informal) all forms of agreement while listening to others speak. This 'yes' shows that the other person is listening only. For example, you may ask someone how to get somewhere using a certain train route. The person you are speaking to will frequently nod and say 'yes', as to show they are listening to and understanding what you are saying', but may then end with,' yes, it is not possible'. So don't let this confuse you