These are crazy to grade from images. The 3 cent silvers are worse. It's tough to see what's going on with the luster in an image. I like the coin.
I saw the flatness on the obverse also. I strongly suspect that is a result of the trouble the Mint was having with the "new" coining metal (nickel alloy). My understanding is that it took quite a while for the Mint to "solve" the difficulty. If you see a soft strike on the obverse it's highly likely you will see problems with the "III" on the reverse. Here is my only example from my type set. It is graded PCGS MS-65. Note that even though the "III" is fully struck the Obverse still appears "soft". And I recall seeing an MS-66 that wasn't this good.
I say High AU to low MS I think that flatness on the high points is just weak strike. If you look at Phototgrade's MS-61 example the obverse ribs are weak\missing on the first I, which, if I'm not mistaken, is where the highest point is on the obverse in Lady Liberty's cheek. The same can be said for the MS example on NGC's coin explorer. Also some flat points on her hair again can be seen on both MS examples from NGC and Photograde. I see luster in the OPs coin...i think the white balance is off on the OPs pics so everything looks lavender...If that were corrected I think the darker lavender areas would be flashy white. Below is a comparison pic of your coin, Phototgrade's MS-61 example and NGC's MS example. Yours is the black background and the photo has been "corrected" for white balance. Next to your coin is the PCGS Photograde MS-61 example and then last on the right is the NGC Coin Explorer MS example. If anything brings this down to an AU, which is still a nice coin, and something I personally appreciate more, I think it would be the darker areas (areas of rub) on Liberty's nose, chin, cheeks and neck. This is where I think the luster is lacking. However, the MS examples have some darker spots in some of the same areas. Great find...I appreciate the odd balls in our past coinage, this is one of them.
No matter the grade, it sounds like you had a great day with your son! Nice coin. I'm in the AU55 camp as well.