I felt so bad for you reading your post. What a disheartening situation.
The first step has to be to figure out what happened. I know you said you put a plant potted with organic soil in the pond just prior. What brand of soil? Maybe we can start there.
Did anything else happen near the pond? Grass fertilized? Any kind of weed killer applied? Do you live in an area where they might have been spraying for mosquitoes or other pests? Just trying to rule out any other toxic event. When all the fish die like that at one time, you have to assume they were poisoned somehow. You don't have any neighbors that abhor you, do you? Did you use any chemical treatments in the pond - algae killer perhaps?
As for your filter and starting over with fresh water, it seems that would be the judicious approach. You'd hate to get it all back together and find there was something toxic in the water. Sun or shade is really not as big a deal as one might think, since ponds do fine in both locations. They do have separate challenges - as you mentioned, you may have more debris in a shady area, but a sunny area may need more creative shade protection.
You'll get through this and we'll all learn something from your experience.