I think he just got an abused gun. The S&W 910 was an economized lightweight 6906 I think. Pretty good pistol in its day. I have the heavier full size 5906 and love it.
If you're talking about the decocker, if it's turned down it's normal for the hammer to drop down when you rack the slide. And at first it is tricky to get the slide back on fully, you have to have the hammer down and press down the three levers one at a time.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek