Tech research disappearing from tree

Currently (0.23), there is few practical difference between sandbox and career mode after you unlocked the whole tech tree. You have all parts available and are free to use them to do whatever you like.

The only difference is that you can still use experiments in career mode, while in sandbox mode all experiments are disabled. This doesn't give you any gameplay benefit, but it gives you the opportunity to read the flavor texts of the experiments performed on outer planets and moons. Some of them are quite interesting and/or entertaining. One EVA report on Eeloo is really cute, by the way.

Currently there is a lot more science to farm in the KSP solar system than you need to unlock the whole tech tree. And considering that it is planned that more planets than just Kerbin, Mun and Minmus will gain different biomes, the science output will just increase. If and how the developers will balance this in future updates (more tech tiers? Make later tiers more expensive? Scale down scientific values? New uses for science points?) is currently just guessing.

Eve has an atmosphere ranging all the way up to around 100,000m with surface pressure being about five times as high as on Kerbin, and almost twice as much gravity (with 16.7m/s² gravitational pull).

Depending on the height from which you're launching, you will need between ~8,000m/s and ~11,500m/s of Δv to achieve orbital velocity around Eve, which should give an orbital velocity of ~3,500m/s.

Eve's escape velocity is ~5,000m/s, so in order to return from a 100km Eve orbit to Kerbin, another ~1,500m/s Δv is required.