Please find my answer inline for your query:
Can i retrieve all sended friend requests?
Ans: As of now, it's not possible. However if it is required then you can maintain it easily in the storage service with repect to user. Let me know in case you need any help while integrating this.
Where i can see all buddy requests with usernames and messages?
Ans: You can fetch users message/request using the API. Please have a look at this link for more details.
Can i cancel my friend request?
Ans: Yes, you can. Click here for code snippet.
If i add to ban list user who sent me fried request. This request will be automaticaly removed?
Ans: Yes, if you block any users then the request will automatically removed.
Let me know if you have any other question, I will be happy to help you!!