This was always possible because GAF lacks TLS encryption (https). And on that note WPA is more to protect from people getting into your network, it's not really content protection, that is usually handled by a higher level network protocol like TLS.
Yep, probably
Sort of, as it would work the same as the first one, mostly by watching your traffic.
Probably not. It's not impossible, just more difficult to intercept your post request as it's being submitted and then resubmit with something else. Not impossible, but just the effort involved is a good deal higher.
No
On Gaf, maybe, because Gaf doesn't use https:// which it really should. The password submission is still encrypted and salted, but the encryption method is usually pretty dated on forum software like this, and if a hacker is going through the trouble to snoop your traffic on a site like neogaf they'd likely be able to get the encryption key, which they could then use to decrypt your password as it's submitted. But, if you've logged in someplace where you're using a more private wifi (say like, at your house where it's unlikely that someone is sitting in your driveway snooping your data), then you're probably using a cookie to stay logged in. A hacker could snoop this cookie as it's passed over to neogaf, but most websites that deal with anything require more than just the cookie to pretend to be someone else, and will require another login.
No, the password is only passed once when you log in. For websites that use https:// this is even less likely, but of course, NeoGaf does not use https://
Thank you for taking the time to add answers. Today is a day I learn it seems