Question: In the prison scene where they laid out the weapons, why was Deadshot shooting the rifle at an angle when he was "showing off" to Flag and Waller?
Answer: The benefits to having the back-up iron sights is if the battery in your red dot go out during a fire fight. There are several gun accessory manufacturers that make angled iron sights. The are used as a backup just in case. Yes he is showing off, and yes those sights exist.
Question: What was the liquid in the big containers that Harley Quinn and Joker jumped in?
Answer: The same unknown acid/chemical combination that created the Joker in the first place.
Answer: Exactly as you said, to show off. He wants them to see he can fire just as accurately at an angle.
lionhead
I am not a gun expert and attempts to search this to verify my point have failed. However, it seems to me that he rotates the rifle to use the secondary sight. While this may be showing off, I assumed this secondary sight was intended to be used for a different range of target, or made a specific type of target easier. When he rotates the gun, it seems he is switching targets. Research has shown that the gun does have 2 sights (DDM4 MK18).
The DDM4 MK18 Deadshot is using has a single red dot sight, nothing "secondary." A gun has only 1 sight, perhaps also a scope, but nothing on the side that requires the gun to be used at an angle. Firing a gun at an angle seriously reduces the accuracy. If there is something attached on the side it's either a flashlight or a laser.
lionhead
If you re-watch the scene, there are very obvious iron sights that he is using that are accessible by tilting the gun sideways. Holding it normal gets you the scope, sideways gets you the iron sights. Not sure what the benefits would be but there are absolutely 2 types of sights on the gun he is using.
oldbaldyone