So, you need to know how to glue up a panel from walnut for a countertop, correct? I'm pretty new, but I can tell you that you need to edge joint them and glue them. Use a mechanical jointer or hand planes to make the two edges you want to glue together perfectly straight, so that when you put them together you don't see any cracks or gaps. Then simply spread wood glue (titebond 2 is the one I use most often) on both of the faces you'll be gluing, ensuring full coverage, and clamp the pieces together until the glue sets. Make sure when you clamp that you use some clamps on top and on the bottom of the panel to keep from ending up with a cupped/bowed panel.
Biscuits aren't necessary, but if you already own a biscuit jointer they can make it a bit easier to get the pieces nicely aligned during the glue up. And epoxy for edge jointing would be a bad idea. You'd end up seeing a clear line of epoxy between the boards! Wood glue is standard for this sort of operation.
I'd think if you design the counter to allow for seasonal movement of the wood you could install the top now. Your piece is quite dry, I think. People have used air dried lumber for most of history, and here in Pennsylvania, air dried doesn't get much drier than 15-20 percent from what I've read! Wood expands and contracts across the grain, so attach the countertop with a fastener that will allow it to expand and contract. If you own a solid wood table, take a look at how it's attached to the frame. Often it's with special hardware that has a slot through which you place the screw. Screw is free to move side to side 1/4" or so depending on the width of the panel, but still holds the panel down. I've also done it with blocks of wood with slotted holes (drill two holes close together and forn the slot between them with a chisel).