By order of priority, Plate Solving first:
I have a camera that is not supported by INDI presently and thus use external software for imaging. Is it possible to use an external image with Ekos and get it to plate solve? If so, what is the process? I can also see this is a means to continue a session from a previous evening, plate solving to visit a location portrayed in a previous image.

AutoFocus:
Is there a way to insert a image from the camera in the scope into the auto-focus workflow if the image was taken manually with non-indi/ekos software? If so, what is the process and image constraints?

I may be off-base, but I believe that using the Load and Slew option should work. You might use the ccd simulator to get it to capture an image to slew to, bu the initial image to compute the target should work from an external source.
Not sure about the AutoFocus.