I use the Canon 14mm on my 5DII and find it almost able to focus on the scratches on my 8" acrylic dome. I doubt the minimum focusing distance is your issue. If the centre is sharper than the sides then it's more to do with the curvature of the virtual image caused by the dome port vs the flat focal plane of the rectilinear lens.
If you have enough light, I would increase your f-stop as far as possible. For critter identification I would find it easier to see the details with a higher ISO and higher f-stop rather than the reverse.