I doubt Steve planned to leave and have Melinda discovered by someone else. That would leave doubt as to whether or not she committed suicide ( which if it had been presumed that she did would have ended up making the life insurance not pay out.) This would have screwed up the ultimate goal Steve had which was to profit from her death. I do think he changed his plan for how the night was going to go when she did not die immediately. Obviously if a real intruder has come in and shot your wife, you are going to be trying like hell to save her unless you can't...or don't want to. Since she was still alive and he had NO desire to save her, he had to render himself unable to provide aid.
I think what screwed up the plan was the gun jam. Otherwise, he or whomever helped him would have shot her a second time and left her alone and the baby in her crib and SL far, far away pretending a stalker did it.
JMO