can anyone explain the difference

1) John arrived at mary's apartment and found the door ajar.
Upon arriving at Mary's apartment, John found the door ajar

or

Upon arriving of Mary's apartment, John found the door ajar


2) The princess decided to marry th prince who was brave
the princess decided to marry the prince due to the fact that he was brave (or)


The princess decided to marry the prince due to his bravery.


which is correct and why ?

Q1.

Upon arriving at Mary's apartment.....(preposition 'at' is correct.)

Upon arriving of Mary's apartment.....(preposition 'of' is wrong.) It would mean Mary's apartment arriving at where John is.

Upon arriving of Mary's apartment, John found the door ajar - given in the answer sheet.

LOL .... answer sheet is wrong.

Q2.

Both answers are not correct.

The princess is marrying the prince 'who happens to be brave' and not because he is brave.

A possible answer - The princess decided to marry the brave prince.

Since the statement should be transformed with due to
what must I use ?

In this case, both sentences are acceptable. (even though it did not reflect the meaning of the original statement. )