I have read that the engine must be raised to replace the pan gasket.
I do not have access to the question about the need for RTV silicone sealer.
If there are 'corners' where the main bearing caps extend below the bottom of the block sealing area, RTV is usually recommended. If the caps 'blend' with a curve, RTV is generally not recommended. Where engines have a separate front cover for belt or chains, and there is a gasket between cover & block, in many cases RTV is recommended to be blobbed onto the spot where the cover meets the block, taking up any void that the pan gasket(O-ring) would potentially not conform to and fill.
It may be possible to snake out an O-ring type gasket, going around the oil pump pickup, but the gasket would likely end up being oiled, and thus might not seal properly, and have more tendency to squeeze out than a completely dry gasket/surface would have.
tom