I am building an l-shape desk using two 98" wide IKEA SALJAN countertops at 1 1/2" thickness. I want to secure the two countertops together using a mending plate to add rigidity, since the ends will overhang the edge of the workbench frame.

Are two 4" x 3" mending plate sufficient for this purpose?

I will be adding additional support past the edge of the frame (which is only 70") using IKEA OLOV adjustable legs, which I think will be sufficient.

I am concerned about the risk of the countertop warping, since I will be putting a lot of heavy objects on it (clamping monitor arms, extremely heavy microscope, computer cases, etc.) because the crossbar is only in the center. Do I need to add additional table stiffeners closer to the edges, or will the single crossbar be sufficient?

I also don't know what kind of screws to use to secure the countertop to the frame. There are large holes on either side of the frame, but how large of a wood screw is necessary (diameter, and how deep).