Hi
I happen to know a bit about projectiles 
Check out my project here:
Anyway, your questions.
Units in UE4 is centimeters, so 1 UU (Unreal Unit) is 1 cm. That means you get the speed in cm/s. Divide by 100 to get m/s
I don’t know enough about particles to guide you on that, but it should be relatively simple to do the tracing with a mesh.
If you have the velocity vector of you projectile, you can use Rotation From XVector(you might have to normalize the vector first, I’m not sure). This translates it into a rotation. Set the rotation of your projectile to this value. It should solve your problem with silly movement