I see these books as a story of the reality of evil and the struggle of conscience. The battle between pride and humility. The concept of death & sacrifice, resurrection, reward, the ultimate futility of evil & the eventual triumph of good. And while no utterly evil person(ie. a Sauron, or Ringwraith or orc) is redeemed, there are several examples of "regular" (free will) people who go down the road towards evil,(aka, sin, tempatation) but are redeemed in the end....my examples would be Boromir, Thorin Oakenshield, Theoden, etc... I think to wonder on an orcs redemption might be going down a bit of a rabbithole...after all, redemption can only come when a person WANTS to receive it... and I'm not sure we know of any orcs who fit that bill.
That being said, Im all about someone coming back from the dark side, ala DV/Anakin (out of context, i know)....but what makes a glorious tale is that there was a big, bad, dark side guy in the first place..
Tolkien didnt set up a religion for middle earth(within the main books) but rather, wove Christian truths into the story- and I think that was intentional.