This issue can be reproduced in a different way with no mods installed: https://www.youtube.com/watch?v=mrb5w-Wi2Ss
Maybe partially. Looks like the sound and particle part are separate issues.
Nope, shaders change nothing about it (removed optifine completely).
This method is never used, so it does not impact gameplay at all. But that is only for the moment, it is quite likely it will be used at some point in the future. It is definitely something which should be fixed, to save some headaches for future versions.