I hate to be blunt, but after reading the previous posts it is clear you don't want help with this.
You only seem want someone to agree with your preconceived notion that it is Japanese and you didn't get ripped off.
Read this slowly and let it sink in: CHINESE FAKE !!!
"What makes you say that?" "I simply want to know WHY it’s decorative"
Obviously not what you want to hear but,
everything about this thing screams Chinese fake!
Blade geometry is wrong!
Hada is wrong!
Bohi and boshi are horrible!
The entire koshirae is wrong, fittings are clearly not Japanese!
You did not provide a photo of the nakago (Tang) or mei but I will bet you $500 it is wrong also!
"Also you can’t tell from pics...but it looks and feels old."
Yes I can and no, it does not!
"the hamon is too uneven/broken to be otherwise?"
It may be handmade or partially handmade, but
if it was, it was handmade in China!
"I think the mokume grain structure is beautiful. You have seen this in low quality Chinese swords?"
Yes, it is one of the first things that indicates a Chinese fake.
"An old man brought it in and said it is from WWII. I think it’s much older."
He was wrong. You are wrong, it's not!
"I mean why won’t I want to take it apart to see what’s there?"
They are just telling you that it is a pointless, waste of time.
This post clearly indicates that you have absolutely no knowledge regarding Japanese swords. But thats ok, let this be a lesson for you. Study, read, learn before you throw your money away again.
I am providing you with a couple of links which may help. Here you can read some basic info and look at some legitimate Japanese swords.
If you would like some help in acquiring a genuine Japanese sword contact me, I will point you in the right direction.
http://yakiba.com/beginner_page.htm
http://yakiba.com/Newlisting.htm