It is supposed to be "mostly" API/ABI stable at the C API level. The C++ API is always (and has always been) completely unstable. One way to address it is to have your own wrapper around LLVM that provide the feature you need, and these wrapper can retarget multiple versions of LLVM at will.
Hard to say. I'm pretty sure you can create any piece of IR (almost), and Optimize/CodeGen. We're open to add new APIs when someone needs one that is missing.
We only break the C API when there is really no choice.
For example when I removed the C++ API getGlobalContext() I made sure to preserve the equivalent C API: http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-201... ; but it is not always possible.
