I think Warner got the rights to the Hybrid Theory EP (and maybe other songs that didn't make it on Hybrid Theory, like She Couldn't) the moment they singed Linkin Park. You guys are forgetting (versions of) songs like Carousel, And One and Part Of Me were found on Warmer Bros. Records demo CDs. Step Up was also released in the In The End single and High Voltage (although a remix) in the One Step Closer single. That leaves only Technique out, but then there was the Hybrid Theory EP reissue in 2001 and the songs have been on the LPU store for years. As for the Xero demo, Rhinestone is Forgotten, Reading My Eyes was registered for release on LPU 6.0 and Stick N Move on LPU 9.0. The only song Warner might not own is Fuse.
I'm not sure what kind of contract the band has with the label, but it seems to me a label doesn't really need a band's approval to use their music. Just look at the "greatest hits" releases by Avenged Sevenfold (Warner) and Papa Roach (Geffen). The bands didn't support those albums at all. The Papa Roach one even had an unreleased track. So I think they can release whatever they want as long as it is in their archives.