First of all, I see a little misinformation about the Forgotten Realms floating around. I hope I can dispel some of it.
The Druid Issue: In the PHB, a druid can worship a Deity OR just the concept of "nature" itself. In Faerun, they cut out the "OR nature" part. I don't see this hamstringing anyone, as the generalized definition of a cleric is a devout representative of a higher power. PHB Druids revering just nature are the same way. So I don't see the problem of contention.
Religious conversions: Religious conversions are common in the real world and in D&D. In fact, I say they should be MORE prevalent in D&D, as campaigns to convert people to a certain religion would make for some good and intense roleplay. There would be nothing wrong with the concept of an agnostic fighter who "gets religion" and decides that he wants to spend the rest of his life fighting for nature - in the form of Mielikki, or Silvanus.
The source of magic in Faerun: Mystra's Weave is the source of power for wizards, bards, and sorcerers. She does not care who uses the magic for what purpose, AS LONG AS THEY RESPECT THE MAGIC ITSELF. It's like someone loans you an expensive car, and they don't care whether you use it to drive to work or to party all night, as long as you don't trash the car. Use it to dominate the Realms, as long as you don't harm the magic or its source.