I will summarise as best I can because I am so bored of this problem!
CX9 2010.
Intermittently wouldn't start in park, if we rocked the car back and forth it seemed to help - until it didn't. It deteriorated until finally not starting at all. Mazda ran a diagnostic. - nothing found. Went elsewhere and had the starter motor replaced.
Worked for about 1 month and then same issue began happening again.
Back to mazda - ran another diagnostic, said the starter motor was faulty. Starter motor replaced again at original mechanic (no charge)
Problem continued immediately. Discovered we could start it in neutral. Did that for about a month.
Now car is intermittently not starting in neutral as well.

Some points - when it is not starting I mean the engine doesn't turn over at all.
-When we do now manage to get it started in neutral if we then turn off the car and then immediately turn the key it will start in park for a time.
-Car has a new battery.
-The key fob also has a new battery..

Suspect neutral safety switch (or transmission position sensor I think is what Mazda calls it.)
Also pondering if the key needs to be reprogrammed. Mazda has said they don't know if they can do this as we only have 1 key and they need both (bought the car second-hand - only ever had 1 key)

Is this scenario familiar to anyone? Any ideas? I don't want to pay god-knows-what for the neutral safety switch to be replaced and find it does nothing.
Videos attached showing what generally happens and then a moment of success starting in neutral - but we get the same failure to turn over in neutral a lot!

 

the circuit should have been diagnosed with a volt meter or at last a light probe when it wasn't starting to pinpoint the point of failure. Shame the started was replaced as a possible cause