I'm sorry it didn't work for you. I never experienced any of the problems you are listing but I can try to help. To fix the shooting you may need to open the Pause Menu (Tab) and change the difficulty, that setting may cause the player to not shoot or deal damage if it loaded wrong.
As far as gameplay I know there's a lot of moving parts but all you need to do is hold left click to deal enough damage for the enemy health to turn green, then hold right click to "eat" them gaining their abilities and stats.
I hope this helps!