Webway portals need things set up on the other side, and were built before the Warp was corrupted.
The Tau, depending on what source you're reading, basically force themselves into the 'space' between the Immaterium and the real world, before allowing themselves to be forced back out like a ball held underwater, traveling large distances in the process. And that's still at least twice as slow as Imperial travel, on average(much safer and more reliable; also, I would say five times slower, but Merchants can achieve at least as much as a third of avg. Imperial Warp Speed, and the newer vessels were designed to be able to go even faster and farther by shaping their hulls to dive better as well). It does, however, rely on the turbulence of the Warp, to some extent. Not greatly, but it helps, and the distances between Galaxies are absolutely massive, which is the real issue.
Chaos doesn't spread because there's nothing in between to feed on, basically, which is what I originally meant to say, I mean.
The Hive Mind has a single soul, sure. One massive, spread out soul. Usually it's called the Shadow In The Warp.
Eldar don't have reincarnation. They die, they go in a soul stone, they help power armor/vehicles/get added to the Craftworld's Infinity Circuit/World Spirit/Slaanesh's trophy hoard because Dark Eldar don't use Soul Stones. Or they get saved by Cegorach because Harlequins. Solitaires don't even have a soul.
Tau have a warp presence, it's just very small compared to humans (or most other things), and they can't be located in the warp very easily; mind control would be difficult, or other such ilk. Fireballs work just fine.