The Criterion version has been long out of print and used copies are astronomicaly high, will WB ever release this movie for the people who weren't able to get it, along with Spellbound?
Well, I don't know that WB would release it or they already would've in their box set. I actually think MGM or someone owned it, but with the Sony buying it, there was a set that was delayed. I'm not 100% sure about it, but I think a rerelease was planned at one point.
Disney actually owns it. It was a Selznick production, and ABC bought out the Selnick library years ago. That library became part of the Disney family when they picked up ABC. The way that MGM got involved (or, at least, that they were announced to have been involved before it was cancelled) is that they have licensed many ABC films from Disney for DVD release. See also, for example, Prizzi's Honor, Charly, SpaceCamp, and Silkwood.
Prior to the MGM deal, Disney licensed many of the same titles to Anchor Bay (which is why there's an older Rebecca disc from them). Disney also licensed some to Criterion for a brief period (which is how the Rebecca, Notorious, and Spellbound SEs came about, as well as Straw Dogs).
Long story short, Disney are the ones to talk to on Rebecca, but MGM may still have a distribution deal for it.