1) As You May Not Know, They Aren't Making XBOX360 Anymore, Meaning That If You Get Banned, It Will Cost More To Get A KV (Key Vault) To Unban Yourself.
2) If You Have DLC On Your Other PS3 That You Don't Want To Re-Download, Then Yes.
3) Yes, Depending On The Menu.
4) You Can, It's Called Bypass, But Only Other People On Bypass Can Join You, And You Can't Join Them.
5) If You Don't Be Stupid With Unfair Aimbot You Shouldn't Get Banned Easy. You Should Stay Away From GTA 5 If You Don't Want To Get Banned, Because It Is Very Easy To Get Banned
Hope I Helped