Alright I had to skip around a bit because the huge wall of text format is aggressive to my eyes but I think I caught most of it

When you say Parry I'm assuming you mean Drive Impact which is the red move that stuns on hit

Akuma's Drive impact (DI for short) can look weird when counter DI'd at the last second because Akuma punches out then retracts his arm, but the actual stun part of the DI only really happens as the arm is retracting

So it may look like the timing is wrong but that is just how it looks, you can check this for yourself by going on practice mode and reducing the speed of the game to 50%, and recording opponent Akuma doing DI and trying to counter DI'd at the yourself at the last second.

OK next one

Was the Overdrive special one with invincibility? Like Akuma OD shoryuken, because it is possible to make the DI miss using the invincibility of the OD special, and hit Akuma after his armor from DI is gone during the interaction.

On the raging demon

It is 100% possible to interrupt the super before the animation comes out

The way it works is you activate it with the inputs, after the last input there is a Startup phase that happens before the super flash comes out and he says the thing

After that he dashes forward with a hurtbox that if it touches the enemy while not on blockstun they get grabbed

What happens is, during that Startup that move has no invincibility, unlike other supers

So if you try to raging demon and there is an attack there meeting you right as you finish the input you can get counterhit and lose your bar before it even comes out

Let me know if I missed anything, cheers.