1.  All Istari are Maiar, but not all Maiar are Istari.  The five "wizards" were a subgroup or "order" of Maiar chosen, apparently, for the specific task of helping the free peoples of Middle-earth oppose Sauron.  Whether or not they had any other jobs or if the order even existed before or after that time is not really made speficially clear in Tolkien's writings.

2.  Morgoth might have been able to make a ring like Sauron's but then it would also have exposed him to the vulnerability Sauron invited in so doing (as would to anyone else who made such a powerful device).  Tolkien says that every act of creation incurs an expense of strength and power on the part of the maker (at least among the Ainur and the Eruhini; Eru Himself is doubtless exempt from this).  In foring the One Ring, Sauron concentrated his power, but he placed the greater portion of his own strength into it in order to make it work as he wanted, and thus if it were to be destroyed, he would effectively be destroyed as well.  By spreading his own power and influence through all of Arda, Morgoth affected more was harder to utterly defeat, but his effectiveness was also spread thin.  The Ring also was meant to dominate, and that is a very negative and evil act in Tolkien's mythology; it does not appear to be possible for good to counteract force of that kind with similar force.  By using the methods of evil, they become evil -- as happened with Saruman.

3.  I'm afraid I don't understand the question.