Stingray
Hello Let us look into your problem one by one
Q1 canon Mark III 5D,did you follow the prompt to set your camera
If the no error with the settings,please submit the problem from the feedback in app when the problem occurs,
BTY ,We've received feedback on 5D Mark III,it works on AAP
Q2 So I tried to use my ASI 120mini as main camera on a ZWO guide scope and then it failed, impossible to focus
Did you add the 1.25 extender?

Q3 so I decided to use my CEM26 instead and then it worked only once, the baud was wrong and can't connect
The CEM26 baud should be 115200,it defined by the mount manufacturer
Also please try to the latest Asiair app, the CEM26 added in the mount drop list