Meh. It worked fine in YJ. Those of us serious nerds would take pleasure in decoding her spells, the less serious would just take it as the language of magic and roll with it.
My first reaction was "hm, neat." But after a second or two, I concluded that look is better for The Enchantress.
Not a dealbreaker, but difficult. Either you get a voice actress who can speak backwards, which is easier than finding an actress who can do the same but also has to physically portray the character, or you dub and edit the sound, which again is easier to do in animation than trying to match in live action. Ultimately it's just easier to get away with backwards speech in animation than live action. It might not be a deal breaker, but it honestly might be a lot more trouble than it's worth just to appease us hardcore comic fans who are the only ones who would care about this. Especially for something we couldn't even tell was backwards speech vs gibberish by hearing it, unless we rewind it with special equipment or something.
I'm not seeing the issue here. How is backward speech any different from the usual spooky sounding mumble jumble that a lot of other magical characters use?
If they can't learn their lines then they are not good actors in the first place. A good actor would just view it as a challenge to get into character.
Also, most actors, especially leads, work with the writers while figuring out their lines. So they will likely experiment with a couple of takes to figure out which ones works and which ones doesn't.
I somehow don't think "can't learn their lines" is a good argument here when their line reading is comparable to learning a whole new and incredibly hard to pronounce language. Probably harder than learning most languages actually. It's why only a few people even do it.
That depends on the lines they give to the actor. They will likely do a screen test. Give the actor a few options and go with the take that works best. It's not that different from the usual gibberish you see in supernatural based shows and as long we are comparing line readings, again, it's been done in animation.