I agree with number 4, but you also need to make sure you enter into an agreement with someone you can trust. Mixing business with pleasure can lead to problems in both areas, but if you don't know your partner well that can lead to problems too. Basically, you should be at least somewhat friendly with your business partner.
I would agree that it is a good idea to keep friendships and business relationships separate. However it is difficult to have a business partner that you can not have a friendly relationship with.
Well, this is a big topic, but my initial thoughts would be ensuring that you have a very clearly defined business relationship that does not transgress into the personal. Defining a relationship in this fashion helps just to manage the relationship and so both of you know your roles. It becomes more difficult if that business relationship turns into a friendship (but not impossible).
It appears that what you are interested in is the ways of communicating effectively with business partners rather than the entire task of developing good business relationships.
Good business relationships are based on delivering value and benefits to your business partners, rather than by relying on just communication or talks. Yes, communication is also important, just as lubrication is important for efficient operation of an engine. But the lubrication must be supported by many other inputs such as good design of the engine, correct operating procedures, appropriate maintenance and right grade of fuel. Similarly in business communication must be backed by many other requirements of effective business. Focusing on communication alone is likely to lead to failure rather than success in developing good and long lasting business relationships.
Having developed the foundation of good relationship based on effective means of delivering value, we also need to develop effective means of communication. The ways and skills of communication applicable for communicating with business partners is not really different for business partners. What chances is the matter being communicated. But the ways of communicating effectively do not change withe the matter being communicated.
The whole field of effective communication is too large to be covered here. Further, it requires substantial practice rather than just acquiring theoretical knowledge about effective ways of communicating. I suggest that best way will be to read a good book on effective communication and practice communication as suggested in the book. One can also consider joining a communication training program.