I think with LAURA, it boils down to a suspension of disbelief. Is the viewer willing to suspend disbelief in certain instances, to continue buying the story as presented? In the case of Webb's casting, I am unable to do that because he is so obviously effeminate. And it's not that I have anything against him personally being that way. But in terms of this role, an effeminate man has much less motivation to covet a beautiful young girl, because the sexual attraction angle is zero. It should have been cast with someone who was straight, or who at least had more sexual ambiguity and whose screen persona conveyed more intrigue or mystery.
The only way this story works for me as presented is if I regard it as high camp, with this version of Waldo jealous of Laura, because he is planning a sex change and wants to take her place. That way, he can wear her clothes, enjoy her position in society and have her men. I don't think that is what screenwriters of 1944 had in mind.
Probably Preminger cast Webb in the role because he knew Webb would come across as a homosexual on screen and wanted to test the code. The production code office went along with it, because in their conservative view, a homosexual would definitely be a societal miscreant, capable of vile acts, like deception and murder. So ultimately, all this film does is reinforce stereotypes.