There are two copies of the arm, so you do not need to duplicate or mirror it. The Arms can be printed on the large flat side but if your layer adhesion is weak the holes can break. You can print the arms on the top thin edge, using a small amount of support for the bit hanging out, this makes the arms stronger.

The arms attach using rivets. You can use a small length of filament through the hole. You can use a soldering iron to form the heads of the rivets. For more information on Rivets see the Rivet Cutter Tool.

I don't know if you can see through these sorry for the terrible remix lol.