Just for clarity (and possibly sanity) are you saying that replacing the sensors did not work (despite the garage trying working sensors successfully ??????? )

 

The only reason it got better was because they remove the mprop (introducing air into the system). But that only lasted for a week. Gradually the symptoms got worse and it wont start again. Yes i did replace the mprop with a new one. Still exactly the same symptom.

At this point the only way to get the engine started is to crank the engine with the fuel line disconnected and hook it back up again (idk why it works)

 

The only reason it got better was because they remove the mprop (introducing air into the system).

That does not make much sense to me. You do not want air in the system. Air locks prevent clean starting.
Anyway, you replaced fuel regulator on the fuel pump but you did not replace fuel rail sensor??

As to your question and video - I have no idea. For a start, fuel rail pressure runs at about 1350 bar (about 20,000psi). Diesel at that pressure can be injected directly through the skin, so I would not be inclined to remove the pressure relief valve pipe in that manner.
(Maybe a replacement pipe into a measuring container to get some context, but I've never had the need to try that.) But then, that does not look like high pressure overflow either.

 

Taking a further look at your video and a closer (magnified) look at your images It appear that the fuel flow you are seeing from the pipe is coincident with the action of the fuel pump. This leads me to believe that the relief valve is stuck in the open condition and is allowing fuel to simply exit the fuel rail without building any pressure. [If it was a case of over-pressure relief it would be more like a short burst of fuel, then nothing, as pressure in the rail equalises and the valve closes.] If the relief valve is stuck/failed open, you would naturally have low fuel pressure (code) and no start.

I believe that Kia will not sell relief valves separately, only the fuel rail. You can get valves on ebay ........

 

No, i didnt replace the fuel pressure sensor yet since i believe it to be still good ( reading proper pressure when i got the engine to start). I took the proper safety precaution when i disconnected the return line dont worry ( even had a fire extinguisher on standby)

Since the pressure relief valve seems to not be working right, i might get a mechanic to verify the fuel pressure during cranking and also the fault code stored. It could also be the fuel rail sensor not detecting the right pressure during cranking.

Thank you for your opinion on this matter.

 

Good News, i think i found the problem