Labcorp drug test help to ease my mind

I'm a habitual smoker - about an 1/8 every week and half, for about a year and half, maybe two years. I stopped smoking for 21 days, and on my 21st day took a pre employment drug screen at Labcorp. I'm 5'7, 170lbs, and am in decent shape - I lift weights and do cardio a few days a week. On my 14th day of being clean, I went to a local urgent care, and paid for a drug test. The urgent care indicated that this drug test was the same as what Quest Diagnostics conducts. This drug test came out negative.

I've also been doing home tests - using the CVS FirstCheck and Walgreens QuickScreen tests. All of these have been negative - albeit with faint lines. I also home tested my first void of the day, the day before my actual labcorp test, and this too was negative (faint line as well).

On the day of my test I drank about a half gallon of water + 32 oz of gatarodate + 1 cup of tea and 2 cups of coffee. I also ate a protein bar. The previous 36 hours before my test I loaded up on calories, trying to 'reverse metabolize.'

My question is what is the likely hood of my actual labcorp test returning positive? I understand that levels of THC can vary day to day. After doing research online, I've found some individuals who failed after 21+ days. Also, I noticed that my specimen was immediately put in closed box, and on my chain of custody form, the MRO address was located in a different state. Anyone know what are the chance of labcorp doing a gc/ms test without even doing an immounassy? Also, even if I pass the immouassy test, what are the chances of them doing the gc/ms? I've read on another forum where an individual registered 48ng/ml on a 50ng/ml test, and then labcorp did a follow up gc/ms test, causing this individual to fail. This individual also had called to follow up on the results, and so perhaps this is why they failed, but I would also like to know if this is a common practice.

I've also been taking niacin, milk thistle, and vitamin C supplements every day for the past 21 days. I'm not sure if these actually help, but I figured anything could help....

Any input would be great, cause I'm real nervous. Would like to know if I should start applying to other jobs right away or not..