Concrete – Where to seam concrete countertop

Yellow line is where you need to go, make the form slightly longer on the meeting edges so the ends can be recut to a clean joint. the reason being, moisture loss in concrete will change the look of the slab were it looses it rapidly. This means at any joint in the form where water can leak out will force the concrete to cure differently. Caulking the corners will help fix the problem an will also radius the edges a little or more if you choose, when done in the right spots. This should be done on the meeting joint, but of course you don't want a radius where they meet, hence the recut. Have some 1/4" stainless steel rod to drop into the wet mix at the sink front to strengthen those ends. If you put any on the back edge, be sure you steer clear of the area where the faucet is drilled.

You may be aware of this already, but I need to mention this. Concrete slab tops are honed/polished like natural stone is. A stain can be added to the surface prior to honing to add a variation in color too. Melamine will give a good start, but the honing and polishing will give a fine finish.

You should be able (with a cheap metal-drilling bit you don't love much) to drill those out and try again. Drill them out, use a correctly-sized masonry bit to drill the holes larger, vacuum all the dust out of the holes, and set new anchors. Perhaps set the anchors a bit deeper to compensate for the slight spalling at the surface here. You can use "hydraulic cement" to patch the surface, but don't expect the patch to take much of the load, so just put the anchors in a bit below the surface in the correctly-sized holes.

If you choose to abandon and move up the wall, that looks like plenty of room so that the old holes won't bother the new ones. Drill the anchors out and patch the whole area with hydraulic cement, in that case.