That's a very perceptive point -- "softening" Sir John, who had cut him off socially since his marriage, was a sort of dress rehearsal for the much more complicated mission of softening Elinor. You're right; W's success in that quarter might've given him just the courage he needed to embark on the daring venture of convincing a sleep-deprived, desperately worried young woman, left by by herself at night with a possibly dying sister, to relent and say she forgives him...