Selcier: You want this installation to meet or exceed the National Fire Protection Association Standards whether required by your local code or not. A competent contractor can meld your esthetic vision with the NFPA. Here is a link that might be useful: NFPA

Those are milling/sawing marks. Two ways to remove them, sanding or planing. Two ways to sand, hand(ROS or sanding block) or power sand in a drum sander. Two ways to plane, hand plane or power planer . The first of each can be done DIY---but that will literally take days. The second will need to be done on industrial equipment, meaning take the piece to a shop with one or the other machine(most shops like that will have both types).

Most codes specify mantel distance from the firebox itself. As a general rule, a mantel is placed about 12 inches above the fireplace opening. Add an inch to the distance for every inch that the mantel protrudes. So, a mantel 6 inches deep would be attached 18 inches above the firebox opening

When we moved into our house, the fireplace had an old, decorative surround/mantel. We decided to replace it with tile around the fireplace and a 6' non-combustible wood-look mantel on top. When we removed the former mantel, the sides and top had large gaps with mortar caked on to the top part of it. Photos: https://imgur.com/a/JRe4gia We would like to figure out the best way to prepare the surrounding area to be tiled, and to allow for the mantel to be installed flush against the surface. We would appreciate any ideas for the best ways to go about this - do we need to chisel down the mortar? Should we fill in the gaps with something? Thanks in advance for any suggestions.