When the possibility of licensing titles from the Capitol vaults emerged a few years back, I provided D-V with a spreadsheet with a full accounting of everything they released in quad, but also everything rumoured to exist, with a note about the source of the rumour, if I know where it originated. So aside from their own vault searching (which is formidable, and extensive I can assure you) they also have a sense of what should, or might, exist.
As for the titles you mentioned above, I'm kinda dubious about their existence, because I think they originate from that August 9, 1975 issue of Billboard that also has other apocryphal quad listings including Supertramp's
Crime of the Century and Bob Marley's
Natty Dread, titles which we know were never released. But I think one of the fun things about this hobby is wondering if (and hoping that) some of these unreleased mixes exist, so I always keep the mindset that anything is possible so I'll keep an open mind until the evidence proves otherwise. After all, we
recently discovered that
Crime of the Century showed up in a 1975 A&M UK tape catalogue, so maybe it was closer to release than we realized.