It's not a problem to raise the hearth above the firebox floor. In fact, in many installations it is purposely done that way to help keep the ashes in the firebox - it also makes it easier to sweep any embers or ashes back into it. In addition, in looking at your pictures it will probably be a tough job to take the top bricks off and you might find some surprises underneath them when you do. If it were me I'd leave them and put the slab on top.

We have removed the old stone fireplace hearth and will be replacing it with granite with 3 beveled sides. Does it look better to raise the 3/4 thick granite so it ends up 1/8 to 1/4 higher than the existing floor or have it even with the floor and forget the bevel on the sides?

Whatever you use around the sides and top of the fireplace, you can use on the riser. I took out the raised hearth for my fireplace and used a tile surround. I also tiled the area in front of the fireplace with the same tile as the surround. I believe that you need at least 16" of non-combustible material in front of the firebox. Here is a link that might be useful: some examples of fireplaces with risers

We have both. We did a raised hearth in our living room. A wood burning fireplace, not gas. Did the raised for a few reasons. Although the fireplace opening is fairly large, I wanted it raised for sight lines. I wanted it raised for the same reason Weedy wrote, that it actually does provide additional seating when needed. The raised hearth also makes access easier for me when tending the wood fire. My kids were young, 3 and 5 years old, when we built this house. Though the hearth could be considered to be a hazard of some sort, it also acts as a "stopper" or sorts, keeping the kids from running around right in front of the fireplace opening. My kids loved the raised hearth; sitting and watching the fires, warming themselves, etc. In wintertime it was one of their favorite spots for sitting and reading. There are differences between gas and wood fires, so your priorities may vary. Upstairs in our master bedroom suite I have the firebox raised off the floor, maybe 12" or so...but instead of raising the hearth too, I built the hearth itself flush with the floor. Up there I didn't want a raised hearth at shin level when someone was moving around in the darkness. We wouldn't ever use a raised hearth up there for sitting, we have a couple of reading chairs set around the fireplace. Though the room is generously sized, I didn't want the raised hearth projecting into the room. Raising the fireplace box itself still gives nice sight lines of the fire from throughout the bedroom area, even when in the shower or tub in the master bath, and the "flush with the floor hearth" preserves some square footage. So for us it was two different rooms, two different sets of priorities. And do realize your kids will be young and reckless for how long? Whereas the hearth will be raised or flush forever.