I stuffed a piece of foam I cut from a old kick-board and shoved them in the back part of the forward end of the coamings to prevent things from sliding backward. Those cabbies are really handy. I'm not sure about plugging them permanently. I'd guess if you ever turtled (not likely) having that flotation that high might be problematic. I'd want to get Graham or Alan's blessing first.
As for oars I'm with Amos. I'm will eventually get to that. I didn't want to cut down the coamings as I might be tall enough to not need to. In the meantime I have a three piece collapsble SUP paddle that worked pretty good in my one time use of it. I tried a regular canoe paddle and it was too short. I'm going to put some clips on the port side of the cabin roof to hold it so it's handy.
My plan when I make oars is to use the DuckWorks carbon fiber connectors to make them two piece. For storage I'm going to put a port in the left aft bulkhead. To store the oars, remove the port, slide the pieces into the aft storage and then once they clear the seat hatch, slide them aft and then close the port. In theory this should work, but I think my oar length becomes limited to the storage locker length which I think is 5.5 feet. I'll measure first.