In the Berman era warp 10 was conceived as infinite velocity. It doesn't matter how silly that is, it's just how it worked.
Except in Next Gen, where they passed warp 10 in "Where No Man Has Gone Before" and "All Good Things" More like, it was infinite velocity for a couple of manuals and one episode.
Warp 10 is a barrier of sorts in all iterations. In TOS, it will destroy the ship, or send you back in time. In TNG, Warp 10 will take you to the origin of the universe, or send you back in time. There should be some kind of barrier. Since warp travel involves moving the space around the ship, and not the ship itself, and considering the distances traveled at any warp factor... There has to be a point where you are theoretically occupying all points in the universe at once, which makes it impossible to achieve. I don't think TP actually went anywhere. He ended up where he started. Whatever happened to him can only be considered in philosophical terms.
No. It didn't send the ship back in time in TOS. That only happened with the slingshot effect/lightspeed breakaway factor ("Tomorrow Is Yesterday" and "Assignment: Earth") or cold restart ("The Naked Time"). It wasn't just because of high warp speed. http://memory-alpha.wikia.com/wiki/Slingshot_effect http://memory-alpha.wikia.com/wiki/Cold_start
...It's just that in ST4:TVH, the Klingon ship does warp 10 when first slingshotting through time. Or more exactly, it does warp 9.8 and then some, before the time travel effect kicks in. But that's not a "time travel speed" - it's merely the speed needed for the slingshot under those particular parameters. On the return trip, the same ship, now hauling along two-and-half whales and lots of water, slingshots into the future at warp 8. Again, being everywhere at once (and thus at the exact same place as something else) is probably only harmful if it involves time periods greater than some threshold time (Planck's time might suffice). Matter isn't "real", after all, but something better grasped through statistics - Pauli isn't angry about two frogs sitting in the same spot, but about two particles having the exact same quantum state. No matter how finite the duration of Paris' voyage may appear to us, it quite possibly was infinitely short after all, by the very definition. Or consisted of a great number of such infinitely short trips in succession, sidestepping Pauli's iron fist. Timo Saloniemi