I remodeled my basement 15 years ago and I didn’t want to put nails or screws into the concrete. I used a high strength polyurethane construction adhesive to attach all of the base plates (PT 2x4) and I have not had a single problem with them. During construction I had to move a couple of pieces and the wood separated before the glue broke free.

It’s concrete, what upkeep does it need other than sealing it every so often?. That company has the entire system- forms, concrete, dyes and sealers. I used their si acryl product but they have some newer products which seal even better. I have had no issues with anything staining it.

Since you are laying the laminate yourself, cutting the sink hole first seems to make sense. You should be able to get a cleaner cut with the laminate trimmer. But I would be afraid of using MDF around water in a self built countertop. If you don’t get it sealed properly, it will swell up and destroy itself with just a little water. I remodeled my kitchen and used this concrete kit from z counterform. It was fairly easy and makes a solid and practically indestructible counter.

Same here. Every time I have taken tile down I just assume I am replacing the drywall.

Anytime you have a miter joint, you need to have each part cut at half of the total angle. If you don’t, then you get your situation. You should have carried over the horizontal rail and mitered it properly. If it was me, I would just cut the little triangle off and move on with my life.