Gunpowder existing shows up in more than just the bomb at Helm's Deep - Gandalf is famous in the Shire for his Fireworks.
Gunpowder wouldn't really contradict the technology level. The technology fits the the Early Medieval period from AD 500-1000. Gunpowder was invented in AD 900 in China. So, if the books fit the end of the Early Medieval period, Gunpower would exist, but be rare, strange, and something only the most knowledgeable know about. This point would also fit the level of armor - Plate was just beginning to appear, but only in small pieces and being very rare and expensive: The only person mentioned to have any referenced is Prince Imrahil, and it only mentions vambraces)
So, Gunpowder could be in, but very rare, perhaps only creatable by Wizards. Small pieces of plate could be in the game (Vambraces, Greaves, possibly helms), but not have a reaction to create them, limiting them to artifacts, making the few pieces very rare and valuable.