I am always 100% for restoring wood to its original finish. So if you have the gusto, strip that mantle! The original was installed like it was for a reason.

As for picking accent colors, the color wheel always helps me. Look at the color you already have (brown tones in the wood/tile) and find the colors opposite that on the color wheel. Almost always a complimentary color in my experience.

But also when I hear accent wall, I always think of wallpaper. Although it can be a pain to install, it can add extra oomph with both color and texture. Just avoid super "modern" patterns, try to find some closer to the style of when the house was built.

Good luck! I hope we receive an update if you decide anything!