The problem with this episode is Rip Torn's alibi. I really did not like the relationship with Rose business. Here is what the writers could have done:
The night of the costume party, Uncle Leon is looking over the guest list. "Oh, dear", he exclaims, "Mrs. Gotbucks is coming to the party. I forgot, I have the Gotbucks pearls and have yet to put them back together!" From a jeweler's valise, he produces various pearls, clasps, strings, etc. He shoos everyone away from the study and tells them not to disturb him. He locks himself in. He sneaks out the French doors, toting the "pearls" he has just shown them. He goes to Freddie's apartment and commits his crime. On the way back, he dumps the paste jewelry he showed everyone into a dumpster. He returns to the house with out being missed. He unpacks the real necklace, which he had all but finished assembling earlier at work. He calls for a martini over the intercom and unlocks the study door. His wife sees him putting the final touch on the necklace, and believes, along with the rest of the household that he never left the house.
After Columbo has been sniffing around awhile, he encounters Leon at his jewelry store. He asks him to look at some pearls Mrs. Columbo's dear departed Aunt Beryl left her. Just a few loose pieces from a long disintergrated heirloom broach. Leon takes out his loup. He tells Columbo they are nice pearls, but pointing to two of them are says they are worthless paste. Columbo says those two pearls are the ones with a hole through them, his are sold. He got the two pearls Leon says are paste when he was poking around in Leon's study. He showed them to several folks in Leon's house, but none of them could tell him anything about them.
In other words, Columbo is telling Leon he knows how he did his alibi, and his family would be easily fooled by paste replicas.