A: If your favorite editor supports Quake3 maps it shouln't be a problem, you will probably just need to add a few shaders to the path and use the latest version of q3map2 (wich you should use anyway if you are making Quake3 maps :D)
If you are still using an old Quake1 editor you will probably have to switch to a more modern editor.

A: No, also the Quake3 source is currently not available to the public. Tenebrae2 is based on the Tenebrae1 source.

A: It should run at 30fps on a Geforce4TI with a 1.5ghz processor and 256 megs of ram.
If you want fancy stuff like light blooms or very high resolutions, you should ge a faster graphics card like a Radeon 9500+ or a Geforce FX5600+

A: Tenebrae2 supports the original Quake3 bsp format. It may require some work to get Quake3 maps working in Tenebrae2 however. You will certainly need to write new shaders (or edit existing shaders) and supply color, bump and specular maps for the quake3 surfaces. If you want to get every last feature of Tenebrae2 working, it will require you to relight the map. Ydnar (of q3map2) did some additions to the lighting stage for us, mainly to support delux mapping.

A: If you mean, "Is it more optimized?" the answer is yes, it also makes much more use of the graphics hardware, using vertex programs, etc.
If you mean, "Will it run faster on my machine?" the answer is probably no as we are making much heavier content so we are getting the best visual results possible.

A: No, Tenebrae2 only supports the Quake3 bsp format. If you want to make new levels in the Quake1 bsp format or use existing Quake1 levels you should use Tenebrae1.