Arthur C. Clarke said that a sufficiently advanced technology is indistinguishable from "magic". Likewise, the technology of a highly advanced human civilization is indistinguishable from that of an advanced alien one.
Graham, your problem is that any evidence disclosed can be dismissed as the product of a sufficiently advanced human civilization, before the disaster. Therefore proof of ancient alien involvement is impossible.
I consider the "lack of archeological evidence of the earlier, advanced human civilizations having gone through the expected progressive development process" to be the best evidence in support of the ancient alien hypothesis.
The implication is that advanced technology of all kinds, possessed by this advanced civilization, arrived already developed, probably from ancient alien sources.