I have that exact set up, and I get no interference. Could your extension ring be mismarked? I remember when I bought my Kenko 1.4, there were a couple of versions. Perhaps you have a different version than I have. My Tokina has the hood attached.
I also had the same problem. I was able to easily remove the hotshoe as well as the plate under it, but I still couldn't get the button to come up. It seemed that the only way to access it was to open the back of the camera, so I gave up and sent it to Canon to repair. However, I think if you pull up the flash with your finger nail while pressing the flash deployment button, you should be able to get the flash open and then fire, which is what I think your immediate need is. Good luck.
I found my receipt. They charged me $246 with shipping.