Not plausible in any respect (SPOILERS)
The idea that a policeman can walk into anyone's house and conduct a search with no warrant is ridiculous to begin with.
The idea that Tony would leave an unlocked suitcase full of money in a bedroom and then ask his wife's lover to go and keep it company is also silly. Why would Mark need to hide anyway?
In fact - why was Mark involved at all? Let alone assisting in an illegal search.
And to release a person condemned to death in order to see if her key unlocked the door was beyond ludicrous.
One gets used to the awful backdrops in Hitchcock films - and the sometimes awful acting (Margot) the total ignoring of police procedures is a step too far.
My feeling is that Tony was far too clever to fall for the wrong key routine - he knew the policeman was hounding him - it would have been more plausible if he had simply gone round the back and let himself in.
I am not sure what would have become of him - the most they could have done him for would be conspiracy to commit a murder - but as the murder was never committed I don't know how that would be regarded in law.
Any lawyers here that would know?