Can you substitute all-purpose flour with whole wheat all-purpose in bechamel sauce?!

All purpose simply means medium protein content (gluten). Whole wheat all purpose means medium protein whole wheat, so YES such a thing exists.

The gluten development is not critical in a bechamel, so whole wheat is perfectly fine. The main areas where you do not want to substitute whole wheat for refined flour is where gluten development is important (namely breads). Not to say anything bad about whole wheat bread but it is NOT the same product as bread made with refined flour.

if it worked, yes...most Whole Wheat flours are too grainy for sauces and gravies, but if it is "all-purpose" whole wheat, it may be more finely milled, and hence would not affect the texture as much.

the main thing about cooking is not to worry. so it didnt' come out like in the picture, so it didn't come the way chef this that and the other makes it... so what? as long as it came out and looks good an tastes better.

It's fine. You may have a darker color but remember to cook your sauce for at least 20 minutes after adding the flour to get rid of the starchy flavor

Your sauce should be just fine. It may have a darker color but I doubt that the flavor will be altered that dramatically.

Yes. You can do that.

Don't worry.

Good Luck!