1)
Why did Elisha not go out to Na'aman?
1.
Malbim: Na'aman should not think that Elisha cured him via natural means or an incantation. He wanted to show that the Navi's decree works.
2)
Why did he say to bathe in the Yarden?
1.
Malbim: Cold water aggravates Tzara'as, which comes from white Lechah 1 . Na'aman should know that Elisha cures an affliction via an affliction - this increases the level of the miracle.
1
Ramban (Vayikra 13:2) - Baheres (a form of Torah Tzara'as) is from this.
