I beleive XP actually installs with DX8. It definitely works, no doubt about that. The reason DX8 for XP is rare for download is probably because everyone is just hosting the latest version.
Reverting back is not really an option. There is a program called DX Buster, but people have reported big problems using it to remove DX9. The only surefire way to remove a DX version is to reinstall Windows and install a lower numbered version. This revert you to the version that Windows shipped with, and you can never go lower than that.
Playing JF-18 with DirectX 9 is quite simple. YEs, you will miss some visual quality, but I think it is worth it to avoid the massive hassles of outdated DirectX and video driver versions.