I agree with you that it is a stretch. However, there is just one more thing, and I just thought of it. If he would have asked Trish to do it prior to the murder, then the audience would know what the ticket meant when it showed him on the camera, and that would have ruined the denouement, which is anti-Columbo plot structure.

I think the "Research Assignment" went more like this:

"I need you to drive past the Speed Camera on I-405 at exactly 3pm wearing this mask. We have a client who wants to get out of a ticket and I want to show that his ex-wife could've been wearing a mask, no proof that it was him".

The mask could've had a few pinholes, just enough to see through for 5 seconds.

I totally agree with you. That is the first thing that bothered me; I rewound to the part where Trish disclosed to the killer that she suspected him of murder. She also added that he used her to kill the girlfriend. Trish said to the killer that he borrowed her car while she ran around on errands doing research. However, why would Trish wear a mask while running the errands, unless he had to be on a site where people could see him, but not talk to him? It doesn't make sense is
illogical. This is the only time I have ever found such an error or unbelievable error in "Columbo". I lov'em and the show, so it doesn't bother me, but it is very surprising.