On the subject of 'the arm' it does look a bit wrong to me, but I have commend Adywan for using a model (something Lucas is no longer brave enough to do) and it looks great compared to the version from the 'lost cut':
Until today I just assumed it was meant to be Pona Baba's arm but ignoring what the novel and script say and any bits of secondary canon, just looking at the film Ponda Baba never actually draws a gun and wouldn't really have time to.
I had a look through and here you can see the human draw a gun, note Ponda Baba's hands still at his sides:
Then we see it is still only the human holding a gun, the next shot after this is obi-wan slashing at the arm:
I think the idea that it is Ponda Baba's arm just came from a continuity mistake in the editing and that it is the same as his sleeve, but really it never could be his arm.
Personally I think it would make more sense if it was a human arm (great possibility for Adywan (his arm anyway) to make a cameo), it's a shame this has only come up after Adywan had finished his edit.
But like I said everything else looks great.