Please, would you correct my mistakes in these sentences. Did I used a comma correctly?
1. Peter was the owner of two companies, which he had inherited from his grandparents.
2. He was a generous and kind person, which could not be said of his brother, who was mean and arrogant.
3. The house was not complete in time, which caused even more problems for its owner.
4. In the room entered the man, about whom I did know nothing.
5. The patient stopped breathing, which caused panic in the doctor.
(or should I write, causing panic in the doctor)
The first three are correct, though 'complete' in #3 needs to be 'completed'.
#4 is not acceptable. You can say:
A man I knew nothing about entered the room.
A man about whom I knew nothing entered the room,
The man, about whom I knew nothing, entered the room.
Into the room came the man, about whom I knew nothing.
In the first two of these, the man has not been previously mentioned. In the second two, he has.
The fifth is acceptable, but I would prefer The patient stopped breathing, which caused the doctor to panic.