Flaws in plot (spoilers)
As much as I enjoy this movie, and they didn't come much darker in 1951, there are too many aspects of the plot that strain credibility. I'll mention three:
1. Neighbors are almost by definition nosey and gossipy. It's not credible that none of them noticed the repeated nocturnal visits by Webb to Susan's house and were not available or were not called to testify at the inquest.
2. It's not credible that Webb could actually toot his horn and flash a light into Susan's bedroom and yet wake only her but not her husband, or that no one on the street saw the light or heard the horn or saw Webb and Susan together outside before the husband stirred.
3. It's not credible that the outrageously contrived scenario on which Webb murdered the husband could fool anyone, least of all Susan, given all that had happened between Webb and Susan to that point.