If you mean the flip out diffuser built into the FL36, this is only for spreading the flash for use with the 8mm fisheye. I do use an elastic fit diffuser sometimes, but I find it limits the range so much, it really is only effective for close up shots.

No, don't use FP TTL Auto all the time. Only outside in brighter light. Use it to brighten up dull weather or to take the harsh shadows out of faces if it is bright and sunny. If you use FP TTL Auto inside where it is darker, you can get strange ghostly shadows and transparent bodies!

So, inside use TTL Auto; outside use FP TTL Auto if you decide flash is needed. Marshal a family volunteer to experiment at home both inside and outdoors. The most important thing where the pics are critical (and I have never done a wedding myself) is to make sure you are not experimenting on the day. Limit yourself to techniques you know will work in a given situation.