You've got a lot of questions. I'll take the first one.
" LZ, unlike those bands, brought old American blues (e.g. Willie Dixon) into the fray." The Rolling Stones were THE band that popularized blues among teenage white kids in Britain. It's why they took off like a rocket. Their name comes from a Muddy Waters song. According to a current radio interview around the song "Scarlet," Jimmy Page says he first met Mick Jagger and Keith Richards at a blues festival in Manchester well before the Rolling Stones were even formed. The Stones were blues fanatics.
Riff based music existed before Led Zeppelin. "Day Tripper" is built on a riff, "Hound Dog" is built on a riff, and it goes even further back.
"Lastly, LZ branched into multiple genres like few other bands." The Beatles were the band known for skipping around genres and making it fashionable to do so, around the time they evolved into the Rubber Soul , Revolver and Magical Mystery Tour era. Zep benefited because the Beatles had primed audiences to not expect a dozen versions of the same song on an album.
Check out an album by The Who called Live at Leeds, and a documentary about them called The Kids Are Alright, or 50 Years of Maximum R&B. The Who set the template for Led Zeppelin, and John Paul Jones has even said so. Watch their 1968 Woodstock appearance and you'll get it.
What Zeppelin had that was different was Robert Plant and John Bonham. Steve Marriott of the Small Faces (check out their song "You Need Love;" you're in for a shock!) and Roger Daltrey of the Who could hit some high notes, but they didn't have the supernatural attack of Robert Plant in his prime. Keith Moon of the Who is a standard bearer of rock drumming, but like Bill Ward he had a loose and jazzy style. No one had heard anything like John Bonham in rock, who was essentially the sound of Buddy Rich in a rock band, with stack amps.
I can tell you more, because I am an old person who has not dwelled on much else all my life, but that's for your first question.