GLEW is available under the modified BSD license. You can link it completely statically and even that does not force your own code into a specific license.
OpenGL ES 2 is not identical to OpenGL 2 but in most cases a program written for OpenGL 2 core can be ported to OpenGL ES 2 with a bit of effort even if the original program was not written with GL ES in mind.
A lot of games simply require at least OpenGL 2/2.1 core functionality and then add additional features depending on what further core functions (for higher versions) and extensions are available. There is always a risk that features are broken in specific drivers. It happens every now and again, even to AAA games. The risk is much less while you are using common functionality. Extremely new extension or completely obsolete ones (like the shader ARB) run a much higher risk.