1) A person is travelling overseas to his in-laws house who live in Mumbai. He stays there for 11 days but in between the stay the person needs to travel to another city (Pune) for 1 day and come back to in-laws house in Mumbai. So should the person pray qasr or full namaz?

2) What will be the case if he cancels the trip to Pune. Should he then pray full namaz?

by

1) His namaz will be qasr in both cities because he is not living anywhere for 10 days or more in one city and
In between traveling to other cities.

2) If it cancelled before arrived in Mumbai then namaz will be fill because from beginning your intention was to stay for 11 days
But if you changed your plan after few days and you cancelled your trip to Pune if again your stay is less than 10 days namaz will be qasr.