Currently known forces splitting at low energies, and hidden 5th force: nobody knows. Physics is an observational science and right now there aren’t any observations that require such observations, but never say never.

Star wars force: come on, it’s fiction.

Gravity incompatible with QM: basically, quantum field theories are developed by starting with classical field theories (say electromagnetism) and doing some mathematical transformations called “canonical quantization” and “second quantization” (these have wikipedia articles). In the 1920s through mid-1940s this worked well for electromagnetism, and made good predictions except it broke down at very small scales, giving “infinity” as the answer to calculations that should have been finite. In the late 1940s a scheme called renormalization was developed, that allowed cancelling out the infinities and getting very precise answers. That was called quantum electrodynamics (QED). Later this was extended to the strong and weak nuclear forces, giving the standard model (SM). That was harder, but same basic idea.

The trouble with gravity is that when you perform quantization and then renormalization, the infinities still don’t go away. That’s what the incompatibility means. There are a lot of proposals like string theory to quantize gravity, but it’s all very speculative.

As for detecting gravity waves but not gravitons, it’s similar to the situation with visible light. As far back as the 1700s(?) it was possible to combine light beams and see interference patterns, thus confirming the existence of light waves. Light “particles” (photons) are much harder to detect and I think this was first done convincingly by Einstein’s explanation of Brownian motion around 1900 (before relativity).

Disclaimer: I’m no expert and I haven’t made any progress in understanding this stuff beyond the handwaving level that you see above.