Designing Kitchens and Bathrooms - Countertop Problems HELP!!

I am updating a countertop on a 1950's era kitchen. The countertop was put in first, then the rest of the kitchen. We had to cut the old one in half to remove it. The countertop is as wide as the kitchen. The walls are slightly crooked 96 5/16" in the rear of the top and 96 7/8" in the front. I do not want install laminate backsplashes but tile the wall. Here is are my questions:

What length should I cut the new top so the space between the top and the wall is small so the tile will cover it and still have enough room to get it into position?

How do I join two pieces of plywood/mdf together to make a 96 5/16-96 7/8 countertop? The laminate I purchased is 12ft long so it will cover the joint. Would this cause a problem in a few years (crack the laminate?).

If you buy industrial board, it will be 49x97, and no splice will be required. I'd suggest you check the corners with a straightedge and framing square so that there are no suprises. You should be able to scribe the ends to match the walls, perhaps leaving yourself with an 1/8" of wiggle room on one or both sides. You'll apply some 1/2" or 3/4" backing all along the edge of the underside of the countertop, and above all the joints in your cabinets so that when you go to screw up through the corner bracing in the cabinets, you will have something solid to screw to. Using a few strips of backing where needed helps make the countertop a little lighter, rather than screwing two solid pieces together which would be pretty heavy.

Thank you for the fast response. I guess I will have a productive holiday weekend after all!!