Your A410 problem really sounds like it might be a defective CCD imager. Canon would normally fix this for you for free, including free shipping both ways.
http://camerarepair.blogspot.com/2007/11/canon-digital-cameras-showing-black.html
http://www.usa.canon.com/consumer/controller?act=PgComSmModDisplayAct&fcategoryid=225&modelid=13390&keycode=2112&id=29819Unfortunately this is only for applicable cameras A40, A60, A70, A75, A80, A85, A95, A300, A310, S1 IS, S60, S200, S230, S330, S400, S410, S500, SD100, SD110
But you may want to contact canon anyways to see if they will help.
As for your SD500 e18 issue,
The E18 error is fairly common. Usually it's sand or grit interfering with the lens extension mechanism. Or the camera's been dropped with the lens extended. Or the camera has been powered on, but the lens had been blocked preventing its extension.
Here's some things that you can do to try to correct it. Have already fixed several cameras with this problem by using these methods:
http://www.e18error.com/repair.html