Dear johny
It is perfectly fine if your landing with a cast ..... :-[ They may question you about the seriousness of the issue ..... and may even wish you a speedy recovery .. ;D. Don't forget to take your medical file with the details of your facture.
With regards to PR address, you CAN give your BIL's address and you can also travel back to US. If I/Officers ask you about this, please don't hesitate to reveal the facts.
You may add that you have some issues to be sorted out back in US. Your cards will reach your BIL's place with in 2 months. A proper & safe arrangement should be made for the cards to reach you at your US address!! Make sure that next time you enter Canada, you do it with this PR card. All the Best ...Have a fast recovery ....
Cheers, aNil
EDIT - Sorry I forgot to mention about the POE.. It entirely depends on your comfort level. You can choose any place as POE ...depending on your BIL's address (I suppose you will be staying there) ..... OR which POE is nearer to you (from US) .... and you need to go back again....... Entirely your call ...... You need to keep your other hand safe ....... ;D