I did not understand well the meaning of questions 9 and 10. A lot of experimental work is often done precisely to find whether a molecule acts as monomers or oligomers. Sometimes the evidence is conflicting. A classic case is haemoglobin, where the binding of one O2 affects the other sites. Another reason for dimerisation is to bring molecules together and trigger an effect.
Some molecules aggregate in larger numbers.
This is particulalry true for proteins with a structural role such as viral capsids. Although crystallisation is a good method of purifying proteins, it's often possible for co-crystallisation of similar molecules. If parts of the structure are poorly defined it may be difficult to locate them in the X-ray map. I enjoy this.