1999 Cadillac Concours OIl Leak

The maniflold could be leaking but mostlikely its the valve cover, and the oilpan has a gasket that goes around the perimeter of the pan, it doesnt cost much to fix this in my opinion.

Yes, but if you have big hands and arms you may have to remove some other things, if you have the money, I would just have it done

Don't try, the oil pan will not come out without the crossover pipe removed, and the crossover pipe will not come out without the transmission removed. None of this will come out, without the frame removed. If you are going far enough to remove the oil pan, you will be benifitted by resealing the lower crankcase half as well. There is not an oil pan gasket for this, although you will remove a seal from it, that was early design, or what the engine was manufactured with. It requires the use of sealant now, GM p/n 12378521.