I have good credit 800 and a great w2 job with 2 other investment properties (in a different state).Should I just accept the 25% down, or will a conventional bank even offer a 15% down?

Unless vets are considered to be so special that credit scores do not matter a 500 score will be no where good enough for any conventional bank loans.I would rent, with luck you may find a mom and pop landlord that does not care, then work to repair your score over the next couple of years.  

My credit card only charges my Netflix and is paid off in full to maintain 3 lines of credit.

Do you just tell the tenant that you have a space they can live on the cheap, no credit check, no background check, LL can raise rent whenever he/she wants, and you need to leave with 30day notice?

They say, " if you can't take the heat then stay away from the fire", in other words if you can't absorb the cost of doing business then you are in the wrong business, simple as that.This is why I can't stand all of the jerks selling people on the concept that people can do real estate with absolutely no money, no credit, and no experience. 

Most people from what I understand do a lease option because they want to fix their credit to or have more time on their current job. 

Are there any known lenders that works with high DTI and a mid 600 credits? 

I have opted for this approach as opposed to the no money/no credit/no job/ help me seller finance stuff. 

The constant issue I have is unlocking & accessing this equity, preferably through an equity line of credit.

Our local contract states that when you request repairs/credits after a home inspection the sellers first response can not be to void the contract.