When I load jacketed .308 rounds I use two dies - Lee Collet Neck-Sizer and Lee Dead-Length Seater. The neck tension provided by the collet die is enough to hold the bullet in a bolt gun. I have loaded flat-based and boat-tail jacketed bullets this way.
When loading lead bullets I use the Collet die, a Lyman M-die expander, the dead-length seater, and a Redding crimp die. The expander is necessary to keep the case mouth from shaving lead off of the lead bullet, and the crimper is just to remove the expansion.
For jacketed bullets in a bolt gun you don't need a crimp usually. For a semiauto you usually do. But to crimp or not to crimp is a hot topic a lot of times
You can use a full-length die to neck size but I don't think it can be used to ONLY bump the shoulder back. There are special dies for that I think. As to bumping the shoulder back...I only do that if the rounds start being hard to chamber...and that hasn't happened yet in like 7 reloadings of the same 100 cases. When I need to I will probably FL-size that time and then go back to neck-sizing.
The mandrel in the neck-sizer is to decap, yes, just like the mandrel in the FL sizer. You will use the neck-size die or the FL sizer but not both. One or the other. Sounds like you maybe have the Deluxe Lee die set?
I do not clean brass at all so I cannot really comment on that but your OA looks good to me.
-cls