Can I dirty flash the ROM instead of clean flash everytime? - You can dirty flash but if anything wrong happen, you must do a clean flash first to see if the problem still there before reporting anything here.
I'm using LS/VS980 and I have some problem... - Remember that you're using international base so you will have some minor bugs (your variant is CDMA specific). For exp you must use 3rd party app for MMS, visual roaming issue.
I dont have 4G/LTE option in network setting? - Delete LteConfigs.apk in system\app (D802/5/6 only) then reboot.
Why my data network doesnt work? Or slow data connection? - Please check your APN settings to see if it's up-to-dated yet. If you need to adjust it, remember to remind me about the new config so I can include them next release
Whenever I select ART runtime, the phone reboot and still using Dalvik? - You're using Xposed and that's the problem. Xposed doesnt support ART at the moment, it will force your system running in Dalvik mode.