Work arounds to this are to do "Overhead" camera view to better align your mouse on the target, ensuring a red highlight circle shows up as you mouse over them prior to clicking. They "should not" be walking as you target them in combat, but out of combat they could still be pathing, so of course to circumvent that, pause the game to better lock on to a mobile target.
There is a menu option to disable screen shake, unsure how effective, as I have never turned that off. This may help you with the eldritch blast issue.
Target snap seems like a a reverse point to what you just had an issue with on not being able to properly target something, but now you don't like it when it makes sure your mouse is on a target. I assume you mean some spells snap and some don't. Again I have not had the issue you are having so I can't attest to those levels of difficulty.
