..."5. When Columbo brings the luggage inside, you can clearly see that the background outside behind main door is completely different than it should be. But this is very common thing in most of Columbo episodes"...

Yes, in earlier episodes I like how in later episodes they use real locations and real houses interiors. One of the few shows that did that. There's even a map showing all the location shots..

https://www.google.com/maps/d/u/0/viewer?mid=1jcy9YcfVzQwjdcn1PkcA9gzHIRAkdLSH&ll=37.6771906516847%2C-112.12997472783742&z=5