Re the teleconverter. You may be able to bypass it by placing some glad wrap between the camera contacts and the teleconverter contacts. Then the camera will think it is just an ordinary T adapter.
Another issue with the Teleconveter is it won't improve your resolution. i.e. it can't improve what is not there. Your ED80 for example is sampling a particular area of sky. The teleconverter doesn't change that, it just takes a section of that area that the objective is passing on to it and blowing it up. By that I mean if your pixel resolution with ED80/camera combo is 2 arcsec/pixel adding a 2x teleconverter won't change that. It will just take that 2 arcsec and spread it out so that you will get an "infered" 1 arsec/pixel, your resolution won't have been improved at all.
I hope this is making sense
I guess another analogy would be if you imaged the moon with an ED80/400D combo and printed it and then scanned and upscaled the image by 2 and printed it you haven't improved your resolution. You have simply taken what you already had and made it larger with a cost to sharpness and detail.