Shower Drain/Pan leaking through ceiling... Need Consult

[Ben Rigsby]

The fiberglass shower pan was installed by a "pro" ... friend of a friend who is an actual plumber. Of course, now he's always unavailable. 

They failed to remove the plastic covering from the pan, and plumber puttied over it. I thought that was it and removed the plastic, reputtied, and put back together... Still leaks. Further work discovered, the O-ring that is supposed to seal the pan to the piping isn't actually touching anything. There's a 1/8" gap! The only time there is a noticeable leak is when there is weight on the pan. 

I have removed the sheetrock underneath, but have no shower pan experience. I thought paying a pro to do it would save me the trouble. Now I regret not going DIY everything my wife and I did does not leak. (side story: the toilets he installed both leaked, until we tightened the tank screws...)

I'm trying to decide whether to keep trying to solve it myself, or call in a different pro. Also, it's MD Covid-19 time so getting a pro maybe increasingly difficult.