1) I don't think there is any difference between the two, as the Unique Constraint also creates the Unique Index
(when u define a unique index you normally specify most of the physical attributes, but am not sure how many do that when creating as unique constraint)

2) It really depends on the queries
(if most of ur qry is like col1=<val> and col2=<val2> and col3 <oper> then, it is better to have composite index. if most of them have "OR" as the condition, single index may be better.

3) Use the explain plan and see whether the particular index is used in the query.
(you may be able to find many other tools, which can also give this details)