Help Planning DIY Butcher Block Desk
I would add a fourth cabinet to support the long run of the top pieces but at about 27" from the end that is in the corner. This would put two cabinet faces at right angles to each other and preclude opening drawers from both at the same time but should be of the most minor inconvenience.

The extra cabinet idea would eliminate any need to attach things to the walls.

My take is that I would set the cabinet faces an inch from the front edge of the top. That will make it look a lot better.

The top is attached to the cabinets by placing screws up through the top of the cabinet from inside the cabinet and into the bottom of the top pieces.

Joining the two top pieces together can be as simple as using several flat metal strap pieces placed across the joint and screwed into each piece up from the bottom. You can get these strap pieces at a hardware store in the same area where angle and corner brackets are found. Here is an example:



(Picture captured from Home Depot web site)