Check on the site to see if HCM accepts Hanoi applications. But also it should be fine as you could claim that you arrived to HCM but then travelled to Hanoi! Or just take a trip to HCM anyway so you are in fact there! I guess its all about the arrival stamp though!
Yes its possible as u dont need to show a minimum monthly income nor be employed/freelancing for minimum period. You just need to be able to demonstrate to the embassy that you are a remote freelancer somehow(supporting client letters/contracts work) and obviously have the required money in a bank account to show to them.