Ok. First, its good to repeat the same exact thing we already know cause everybody has read the post you're talking about but it wasnt the question. His question was about Thor, which kamihime would be good with her and if Gaia would work aside. So what you're saying is irrelevant.
Second, i don't have any personal opinion about lightning because i only got the 2 SSR this morning and couldnt test it yet.
At last, instead of claiming things you've read like a parrot without having the occasion to test it yourself (nothing personal, you're not the only one), it would be nice to take few things in consideration :
-What you read was true at the time it was written, it doesnt mean its true on nutaku now, or still true on dmm. For example, apparently (and i say apparently cause its something i've read, just like you), at some point, Light becomes the strongest element (or one of the strongest) because of a Kamihime with a unique Light resistance debuff. Maybe next patch, Dark will become the strongest element because of a god tier kamihime release, you never know.
-Its also only true if you have the good kamihime. What you're team will look like is heavily luck dependent. Someone who only has acces to SR doesnt give a f..k water is super op with the good SSR combo.
Aside that, yes, i agree with you, water has a good unit pool, and if you're lucky enough to draw Chtulu when she will be release, i'm sure you can faceroll anything you want.
Its pretty simple, there are 3 Souls who does better than the others : Mordred, Andromeda and Siegfried.
Mordred is really good early game, but the main reason you're choosing her (debuff) loose its interest late game because a lot of kamihime does the same thing but better (for example, Belphegor has a more powerful version of her atk/def debuff)
Unlocking her isnt a loss though cause, as you said, Black Propaganda will be a powerful ex ability lategame, which will mitigate overdrive damage from the boss.
Andromeda is a soul everybody will unlock at some point for 2 reasons :
She is amazing, nothing much to tell. During raid everybody will benefit from her healing and she will rez you're kamihime (you can easily be os by a multihit). As long as she is alive, your group is alive.
Healer is a crucial role and you don't always have access to a good one (the luck dependent i was talking about)
Siegfried just hits like a truck on stun and will allow you to max your chance of getting MVP during raid, if played correctly. Also she will allow you to quickly deplete rage bar, when the boss is the most dangerous.
Whatever you choose between those 3, you wont be wrong.
About EX skill, it will be related to your actual team. Do you need more debuff? An attack buff? Maybe more hp regen or a damage skill? It might change every time you replace a kamihime. First thing you have to look at is "does this buff/debuff stacks with the one i already have?"
I will resume : each type of spells have 3 different frame (A, B and C). Same frame wont stack together.
For example : Dar't, Belphegor and Mordred's debuff (atk+def) are all B frame.
This is a link to japan wiki if you want to learn more. You'll have to use google translate.
Also you don't seems to be far in the game, so maybe before going further, you want to read this.
Damn i wrote an entire novel again :(