A million-dollar wound or Blighty wound is military slang referring to a type of wound received in combat which is serious enough to get the person sent away from the fighting, but is not fatal, nor will it leave the person permanently crippled.
In conclusion, Benadryl does not cause kidney damage directly, but if you uses excessively over a long period of time and there is urinary retention, the chronic urinary retention could ending producing kidney failure.
Benadryl is an anti-inflammatory drug, so yes, it should help. But as anything, it's never guaranteed. Take it as directed, and hopefully it'll work. But if it doesn't, talk to a doctor. Inflammation can be a sign of many things, most commonly are allergies and colds.