To load canvas on my 3800 I use the front feed. I put a piece of fairly stiff paper such as Silver Rag or Exhibition fiber on top of the canvas sheet with the leading edge of the canvas set back about 2". I then feed both sheets in the front until I can feel that it has passed the area where canvas would normally get hung up. Then remove the top sheet and continue to feed the canvas in so it lines up with the guide line on the front feeder tray. After the initial loading, proceed as normal with the appropriate platen gap and paper thickness settings for the thickness of the canvas you will print.
Regarding the HFA paper, I have had few problems feeding from the rear sheet feeder but it is all about feel. I usually use one hand to put a small amount of pressure on the top edge of the paper while the printer grabs and loads the paper. It is very important to put very even pressure on the paper so you don't have more pressure on one side of the paper. For 17" x 22" paper I lean over the printer and put the slight downward pressure from two points on top edge of the paper. With a bit of practice using this technique I have reduced the "paper skewed" errors to a minimum. Let us know if you are able to reduce the amount of times you get the error.