You don't need a lock washer, and placing it between the two struts would do nothing for you since neither piece is rotating. A lock washer goes between a fixed and rotating (bolt) element. The normal kind with the little steel spurs is a like a reamer when you undo it, given that rack components may be aluminum, I prefer blue locktite. It works well and is removable easily. Be sure that when you source a longer bolt that it is a good grade bolt. Normally blue cap screws are a good grade, and they don't seem to rust up all that quickly. A little wax on exposed bits, or oil may help.
When it comes to which piece to sandwich over which piece, it depends on the structure. You want the rack as close to the eyelet as possible since it's the heavy one. In some cases the back of the rack may be flush while the front is not, and the only place you can get the fender is between the rack and the eyelet, but this is not the best situation. Either way I like a really strong tab on the end of the fender struts, and so I weld or braze some flat stock onto there. In the ideal, the front rack will have an eyelet to which to attach a fender strut, or will support the fender directly, and you won't have to attach the fender to the fork eyelet at all.