Ugh. Well, guess I'll just stick to earlier editions -- I care too much for consistency to adapt my interpretation all the time, especially when the Chamber Militant fluff is what I "grew up" with and am used to.
As for the Tau, turns out the 4E bit is not a contradiction: The BFG article for Tau Fleets goes into some more detail, explaining it as a consistent development that basically had the Tau use slow "gravitic drives" first, then experiment with a salvaged Warp engine (which they could only use to "dip" into the space between realspace and Immaterium), and finally they combined this engine with the existing gravitic drive to extend the time a ship can spend in this in-between, significantly increasing the range of their ships as they are now effectively "surfing" on the acceleration wave created by the transition.
[edit] added excerpt: