yes, roundover as much as you want. I wouldn't use the sheerclamp for any kind of anchor for screws through the deck or inside as the clearance to not go through the sheerclamp at an angle under the deck is tight. I've picked up a few CLC kayaks where the sheerclamp corner was not rounded over at all and the sheerclamp presented a sharp epoxied edge on the hand when finding a grip.

You don't need a router, use a plane before putting on the sheerclamp. Not sure of the size of the sheerclamp but the 3/4" width is primarily to satisfy the installation of 1"x#10screws for deck hardware. The MillCreek 13 and a run WestRiver 162/4 were sold with 1/2" wide sheerclamps and as long as you aren't putting screws into it it's more than wide enough. For reference the Arctic Hawk kit only uses two piece of 4mm ply (3/8"wide) for a sheerclamp. I built a s&g with a sheerclamp that was 1/2" wide and the inside edge taken down to 1/8" under the deck and at the side panels leaving a triangular shape.

Just make sure to coat the sheerclamp with adequate coats of epoxy. I've seen a few CLC kayaks where the sealing was inadequate at the sheerclamp and when the kayak was stored on it's side with standing water it found it's way through the seat anchor screw or gaps in the joint leaving some waterstaining on the outside.