The staff looks like a Timeless Aeas Hand.  

@Jarkes
 
Description if the image on DA is as follows: - Not part of Luna’s Shadow Army, nor the Royal Guard of Celestia, but standing amidst both. Twilight’s Sentinels move between ponies, walking freely amongs the shade, and reflecting brilliant light. Outside of combat they are normal ponies who live love and laugh just as all…. But in the throes of war, they have sworn that Equestria will Prevail.
 
So he fights for Twilight, explaining the cutie mark. As for why he looks like Shining, I think it’s just the lightning. In other work, he doesn’t look (as much) like Shining:
 

 
It’s really mostly the mane style, horn, and blue in the hair.

Is there a reason this guy looks so much like Shining Armor and has Twilight’s cutie mark in several spots on his armor and in his weapon?

It’s very well made.

Damn I love barding.

 
Luminous?

I thought it was a very feminine looking Shining Armor.