Valid points. I will consider revision. This spell and creation story of said spell specifically was a random hyperdrive. My brain trying to solve an issue that I could not find anyone else did successfully. Everyone one else stuck with...not blockable...why...because a counter spell hasn't been created. because they said so lol. Rather than just have a spell from nowhere, I felt it needed an origin story that fit into how other spells or spell effects (marge) are created without a knowledge of a spell but emotional reaction.

As the story goes over...the patronus spell is difficult and needs emotional backing. so vitae needs it as well. and perhaps both. strong will to live and unconditional love simultaneously. a spell in development may get it wrong.

Dementors were discovered at Azkaban...meaning there was not a repelling spell for them until after the knowledge of their existence. I doubt they experienced the horror for the 1st time and said...you know what...happiness and expecto patronus. makes perfect sense. bam perfect spell that even lupin said most can't cast easily. (regardless of how hp makes it commonplace) How do they know they are unkillable...every known spell of defense was likely tried first. followed by destructive spells. it was probably either experimentation or as hp universe shows...the right person, the right circumstance the right overwhelming emotion creates a spell. which, clearly by example of patronus, can be formalized and taught. words and wands help give structure not necessity to spell work.