Does it still detect a monitor when you have everything disconnected other than the mouse and keyboard? Try starting the iMac in safe mode; hold down the shift key during boot and login. Let us know how it goes.
Do you have Internet Sharing turned on in your Sharing System Preferences? If you have it on, the Mac is set to never sleep. There are other things you can try on this Apple Support document; but the first one I mention is not directly listed. Apple: If your Mac doesn‘t sleep or wake when expected
I have had this happen on my EOS 40D. Check the contacts on both your lens and your camera body. Make sure that those brass metal pogo pins on the body are working and that both those and the flat brass contacts on the lens(es) are clean. My old standby is to use isopropyl alcohol and Q-Tips to clean the contacts. (Just be care not to drip any inside the body or on the lens element. The Q-Tip just needs to be damp, not soaked. I usually squeeze the extra off by pressing the Q-Tip on the inside neck of the bottle.) Also another is to always change your lenses with the body and lens pointing downward. That way if there is any debris or dust on the lens or between where the lens and body meet, it won't drop down into your camera and get on your camera's sensor. Let me know if that works. Also, try the switching the MF/AF switch on the lens a few times to make sure that it is properly on AF.
I would definitely go to Apple and have them run their internal test software to see if this is affected by the recall. Do it soon, I'm sure you will, because the date at which this extended service program ends is the end of this year, 2016. After that, they won't replace it for free.
I use rubbing (isopropyl) alcohol and Q-tips. [Edited for more tip] Oh, and one other tip is to get a very soft tooth brush, preferably one of those one with the uneven bristles. Those kind of ones that you get for free on the airplanes are perfect, so if you see something like that in a dollar store, that'd be the one. I wouldn't be using an Oral-B brush as you might brush off something from a circuit board. Then for those areas that are suborn, you can soak a Q-tip and then press the Q-tip on the area to clean to help the alcohol soak in. Then you can use the tooth brush to gently brush away the debris and not get cotton stuck to anything.