1 1/2" screws are barely adequate. 2" are good. Before the patch goes on, before the cut is made, set the saw only as deep as it needs to be to cut through the siding.After the cut is made, look for wire and waterline, gas, etc. Mark these on the face of the surrounding siding, you're going to paint it anyway. If the walls have plywood or diagonal T&G behind the siding, check it for rot or mold. I've seen houses with and without. This would solve your end joint support problem for the patches if the underlying wood is in good shape.
If you countersink the screwheads into the wood and fill over them with a caulking, the galvanized screws will be well protected.
If the siding is not as thick as what you have on the house now, you will be able to see the joints from a mile away. You may be able to build up the back with folded tar paper, or you may have to shim it out.
I thought you were doing this?