I don't think putting the temp probe in water vs dangling in the air vs attaching to the side of the carboy would make a difference in the way your swings are happening.
Sure my compressor would not turn on right when I opened the door if I had the probe in a jar of water.
We would get more accurate temp readings for the beer with the temp prob attached to the side of the carboy, but don't think it has anything to do with the large swings.
Right now I am just concerned about stable temp, so I just have the temp probe dangling.
Still don't know why you would be having the wild swings unless that aSD cycle time needs calibration, or:
Looking at your post, the dif should make the compressor turn on 2 degrees above your set temp.
Don't know how your temp is set at 64 with a 2 dif, but you said your high is 64 with a low of 58.
Is the controller set on heat instead of cool? I have the same setting and mine turns on at 68.
hopefully someone with some experience will answer this.