This cabinet is the 612C variant.
1) No you don't need a tube the port is 3/4" the thickness of the plywood.
2) That is a 3"x 3/4" wide brace that goes from the front to the back of the cabinet..I have posted some pictures of my 612C's on this forum somewhere I'll see if I can locate them.
3) That is also a brace on edge diagonally from top to bottom corner and it is reversed on the other side of cabinet.
4) Battleship gray should get you there,or pretty close.
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