Using Django to send emails with AWS with a reverse DNS setup to point at a different server
I'm not sure I understand why you're trying to do this, but if your current email server is operating correctly, and you want to be able to use it to relay emails from your AWS instance, I would simply set up a VPN between AWS and your current network, and allow your AWS instance to send email that way. You're not using your email server as an external relay, so deliverability shouldn't change.
But really, the correct answer is to use SES to send emails. As long as you're not doing anything abusive, you should have no problems with deliverability.
Edit: The specific exceptions you mention (SPF_SOFTFAIL, RDNS_DYNAMIC and HELO_DYNAMIC_IPADDR) may not go away even if you set up reverse DNS for your AWS instance public IP - you are still drawing from a pool of known "dynamic" IP addresses - AWS publishes their list of IP addresses here - and I doubt may people bother to check to see if some small number of those is set up properly.
Setting up SPF to allow AWS to send on your behalf is pretty easy.