Connect Unistrut/Superstrut to the garage ceiling
I would recommend that you don't use a washer on the outside of the Unistrut bottom that has any chance of wedging the Unistrut open.  The fender washer on the inside is definitely the simplest solution, and it has the advantage of leaving all of the unistrut opening clear for your platform hangers.
The strength of your lag-bolt connection is based on the surface area of the threads.  Therefore the length of the bolt is potentially just as important, if not more so, than the diameter.  Remember, you have 1/2 or 5/8 inches of drywall that isn't giving you any grip, so your 2 inch lag bolt is only holding on by 1-1/2 inch or less.  Your joists will be likely 8 to 10 inches tall.  5/16 x 4 inch lag bolts will give you more holding power than 3/8 x 2.