I just do not see where you will save any effort nor any time by trying to get the lower engine case off with out pulling the engine. It is tough enough to get off with the engine out and on a stand.
The rear seal is just a length of braided stuff that needs to be trimmed to fit in the rear bearing cap and in the block. Problem there is that the lower engine case has to come off and the crankshaft needs to be lifted to get to it. There are two pins that poke into the seal, one is easy, it is in the bearing cap. The other is in the block. Front seal is accessable from the front of the engine.
I think I would look at least look at a new oil pump and chain while I was in there. With what you have I would seriously consider having the crank turned and oversize bearings. Nothing less than a crank polish and a new set of main and rod bearings.