Oil pressure light came on after oil sending unit replacement. plz help on 2000 Mercury Sable

I just put my car into the shop to have the lower and upper intake manifolds repaired. while doing the repair the mechanic noticed leaking oil from my oil pressure sending unit. I had him replace the unit and degrease the engine around the oil leak. After about 24 hours my check oil light came on. It has not done this before.

I checked my oil levels and the oil was clean and up to level ( I had an oil change done with the pressure sender.)

It was about 95 degrees today and i only noticed it when I parked the car. It wasn't steady but more of a faded flicker. My car temp gauge stayed normal. Im not sure what a bearing noise is but it ran smoothly and barely made any noise. the rumbling from the air leak was gone also.

1 could my low pressure be from my oil sending unit not being primed?
2 could the oil light be from how hot it was outside and the fact the work was just done, or the weight of the oil?
3 I was told the sending units are notorious for having a shorted wire or not being installed correctly. Could this be the reason?
4 what will the repairs cost me for the above?
5 and last but not least. with my oil light coming on after the pressure sending unit is done should i take it back to the mechanic and ask them to fix the issue (again never happened in the 2 years ive owned the car.)
is their any other advice you guys can give me.

Asked by for the 2000 Mercury Sable