Shell

This is for sure a very difficult shot. You used a wide angle lens with your extension tubes and f/16. Unless you can get f/128 or f/256 I don't see this getting any better. The larger the format again you run into your normal lens with it being for 4x5= 150mm and 8x10= 300mm. with bellows extension and a very very small f/stop. I don't think you can still get it. The background can always go blur but nothing on the shell can, it has to be tack sharp. Darkroom tricks can not improve sharpness over and above the film and developer. Try Acufine it is a fine grain developer and I think better contrast. Rodinal just gives you sharp grain and in some situations is good. but, not made anymore is Microdol-X which I think was one of the best out there. I only have 8, one gal. packs left. I have been using acufine and saving my Microdol since I'm using it mostly on Tri-x and digitally enlarging the negative at this time.