I'd say the "dodgy" Zep stuff was mostly on the debut pre riches/fame, Dazed and Confused is clearly based on Jake Holmes I'm Confused(which Page had covered live in the Yardbirds previously) and should I think have given him a co writting credit although the whole instrumental mid section(which is more of the song) is original so Page obviously deserves credit as well, Black Mountain Side is clearly based on Bert Janch's vetrsion of Black Watershide BUT thats perhaps an issue that arrangements cannot be copywritted, same with Babe I'm Gonna Leave You and Joan Bez's version.
Things like Whole Lotta Love I think people mischaracterize what was happening, it was Plant quoting some Blues lyrics and melody in a track that was otherwise original, the iconic baseline/riff is not at all like WIllie Dixion's You Need Love. That kind of thing was happening a hell of a lot, indeed happened a hell of a lot in blues music before it.
The Spirit case I think is another one of a scumbag lawyer getting involved with the money grubbing descendants of the original artist hoping a pretty incompetant and pro copywrite US court system would help them. He's a bit of an asshole but the whole Robin Thickee/Marvin Gaye case was clearly nonsense as well, writing a song which is deliberately "similar" to a previous song should not mean your liable if they do nto actually share direct musical similarities.
You do have legit stealing of course like Coldplay nicking from Joe Satriani, if your kind you could claim that was subconcious but honestly I think they probably thought they could get away with it.