1. Do I really need upload back side of my photo with signature?
No, you just need to scan the front of your photo.
2. In the 'Other Activity (Excluding Education and Paid Work)' section, do I really need to put all my trainings, travels or voluntary work?
You need to include any substantial gaps between your study and paid work - travel, voluntary work, parental leave or periods of unemployment etc.
3. I can't accommodate the 'Intent to Reside in Canada' letter within 1000 character. Is it okay if put a summary of my letter there and upload the entire letter in the documents section?
Yes. Sounds good. I did the same thing. Good luck.
Appreciate your prompt support