This answer is based on my own experience and might be completely off!
From my experience I can say, that scopes do not influence the trajectory, but only make it easier to aim.
For example, take a sniper scope. Some of them have dots with the numbers 2, 3, 4 under the cross. If you now shoot at an enemy in a distance of up to 100 meter, you can see that there is no bullet drop. If the enemy is up to 200 meter away, you would need to aim with the dot with the number 2 next to it to actually hit the enemy. This means that for each 100 meter distance, the bullet drops by the distance between each dot in the scope.
Onto the close range scopes. I prefer the HOKV x1 scope as it has this red dot in the middle and no cluster around it. On short distance the bullets hit "exactly" where the red dot is (there is still weapon spread and other stuff making it not so exactly). Yet over greater distance the bullet still drops and the spread kicks in harder.
So overall, are only a visual help for the players as its for great distances hard to even see the enemy without a scope. They don't influence the trajectory of the bullets in any way.
The trajectory itself can't be influenced by any weapon modification, only the behavior of the weapon itself (recoil reduction through stabilizers).
The Digital Threat highlights the enemy's character model red which in regular circumstances is not exactly groundbreaking as Apex character models are designed to stand out and be visible when compared to that of PUBG (not to mention Escape from Tarkov).
The important thing is that the scope's highlighting effect works through Bangalore and Caustic smoke, being especially useful in Bangalore smoke, as people tend to stay IN it, rather than getting away from it. Shooting at players hidden in or being hidden in smoke yourself can be game changing if you have digital threat scope.
Best Answer
The gas trap definitly affects team mates movement speed, though not in a good way
As far as I can tell, it does not deal damage. I actually doubt that friendly fire is in any way possible (maybe through Bangalore & Gibraltar ultimate). It also reduces vision, just like Bangalore's smoke grenade.
As I don't own Caustic, I can't tell if a friendly Caustic is affected the same as any other friendly character. Also I am not certain about Caustic ultimate gas grenade, yet I would assume it to behave the same way
This is all based on my own experience within the game.