I agree with everything you said, my main point is that it does not look like a fake. The helmet looks period and that liner wasn't worth the comment. I never ventured into anything else other than my own thoughts . How have you been?

How tall is the wolf patch?

Helmet I posted from a WIA Marine   https://www.usmilitariaforum.com/forums/index.php?/topic/144261-if-only-it-could-speak/

Also this site mentions a short-lived MK2A1 which yours could be.   "This is a little known version of the practice grenade.  Not listed in any of the manuals it can only be assumed that its service life was extremely short.  It is noted as using the M10A3 igniting fuze."   https://tgrm.foxed.ca/Americangrenades/Mk II/The Mk 2 grenade.html

My two cents: First, unless there's a way to trace back and verify the story, it's just that: a story.    Second, the liner has been added post-everything. It would not have been worn with that helmet. Who knows why it was added. People do strange things all the time.    Third, the helmet does look like a round or a piece of shrapnel hit it. Particularly with the smaller entry and larger exit.    Fourth, the Navy often kept helmets topside, particularly in hazardous areas.    Fifth, my theory: this helmet, if the story has any truth, may well have been a helmet kept topside on a ship and was damaged by some kind of flying object.    Could the flying object been a projectile from an enemy plane? Could it have been a round from another enemy unit? Could it have been a round from a friendly ship? (This happened more often than reported.) Either way, it's entirely possible. But, without solid provenance, it's a "wartime helmet, painted in a Navy grey color, that appears to have been damaged by a flying projectile that entered through the top and out the rear of the helmet. The liner was added post-war for some reason." Hope that helps!

Army style were more a russet-colored leather and the USMA were a cordovan (dark brown) color. The USMC version also has a small brass grommet in the tip    ARMY   

I would disagree with it being a "grim task" It's the preservation of history. Of the 140+ WW2 military crash sites I have been to many have had shards of bones from pilots and or aircrew scattered around. At one crash site I uncovered the skeletal remains of the navigator of a B-24, It wasn't shards, it was a person with his dog tag. After reporting it to the coroners office JPAC came out and did the full recovery. They recovered hundreds of bone shards from the other 9 guys and interned them at Arlington. It wasn't a grim task, it was closure. The same idea is true with the helmet. It's a physical reminder of how brutal war is and how 1 inch was the difference between life and  death. Those guys brought those items home as a reminder of how close they came. Survival can be grim and brutal and savage. We need to remember that. Just my two cents. 

I agree, it's not a M21 also because those have rounded fragment edges, not as flat at yours.  And the yellow is not the right color.  I'd strip it off.    On the fuses, I'd say "if the fuse fits, wear it."  In other words, these were made to be loaded, practiced with, reloaded, and that repeated over and over for many years for Boot Camps, or other training facilities.  Whos to say the "last time" before it went to DRMO or salvage they weren't using whatever fuses fit the threads and set off the charges?  The body might have been 10 years old by then.      

The A-washers and sweatband look brand new, looks like war era suspension with new A-washers worked in. Props if that's the case, preserve what you can and let the pot tell the story.

I know that the USMC depot made some 1910 pattern scabbard covers for the long 1905 bayonets. I believe this is 1940 or early WW2. I wondered if anyone has pictures of one of these scabbards and knows anything more about them.