1) Even so, facing fears is not the same as removing them completely. It's only repressing them. And as that one Chozo said, "Repressed fear will grow inside you, even if you are unaware it exists".
http://metroid-database.com/manga/?vid=13&cid=64#manga_top (Page 3)
And no, she doesn't kill Ridley. He obviously comes back for Metroid/Zero Mission with Super Metroid being his actual death. He even states in the manga that he has a revival mechanism where he eats the flesh of corpses.
2) It was introduced in Zero Mission's time. Which means the only games it could have been triggered is Zero Mission, Prime 2, Hunters, Prime 3, and Other M.
Zero Mission, we have the expression as you said, but as someone pointed out to me, it doesn't really seem much of a shocked expression. Also, at this point in the manga (which covers Zero Mission up to the final fight with Mother Brain), we see that Samus is more determined to kill Ridley than anything else, putting her fear aside until the job is done.
Prime 2 and Hunters doesn't even have Ridley, so point is moot.
In Prime, Meta Ridley wasn't "killed", he was repelled. Granted, he did fall without working wings, but he's survived worse. Him coming back would not be any sort of surprise, hence no PTSD in Prime 3 when he returns. Nor when he comes back as Omega Ridley later, due to him coming back through the power of Phazon, and by this time, Samus is well aware of what corruption powers Phazon has. At this point, it'd be hard to tell if it was really even Ridley in mentality fighting her at that point, or just some corrupted puppet of the Leviathan on the Pirate Homeworld.
Now, by Other M, Ridley is supposed to be dead for good. There's no possible way he could survive an exploding planet (at which he didn't to our knowledge). Samus could finally put her mind at ease in regards to him, knowing she would never have to see him again. And then BAM! SURPRISE! Here he is again without any warning! Cue repressed memories flowing back at full force.
3) It
can, but in this case, Ridley is specifically the trigger. Note, that just because something CAN do something, doesn't mean it automatically DOES. For example, a war vet in Vietnam saw his comrades burned to death. Grilling on a barbecue reminds him of that and triggers his PTSD. While it's not strong enough to affect him too deeply (which the only thing that would probably do so is seeing people be burned to death again), he does mention that it feels different to barbecue ever since.
If Samus saw like, a creature that looks a bit like Ridley, or someone yelling out what Ridley said as he was trying to kill her (but instead killed her mother), it would probably trigger it as well, but it wouldn't be as strong as seeing Ridley
himself after he's supposed to be deader than dead.
4)
http://metroid-database.com/manga/?vid=13&cid=62#manga_top (Page 5) Translation directly states "symptoms of PTSD".
5) You do not understand how PTSD works. One does not "deal with it" or "recover". Once it's there, it's there to stay. Now, one can work with keeping it under control, which Samus has managed to do.
However, she relapsed (which can very well happen IRL) when Ridley, who's supposed to have been blown up on Zebes, is somehow back out of nowhere without any logical explanation, causing repressed memories that she thought she could put at rest to hit her like an 18-wheeler.
6) For the time being; as I've said, one never gets over it completely.