The private codes or PIN codes are only available to the ones that got the code. Either through email or regular mail.....but yes they are one time use and for the person they were addressed to.
No one really knows how you get more of those. Some people register and 'save' trips on the Disney site and others order the free DVD planning guide from the WDW site. There really is no method to who they send the PINs to, but those are some things you can try
As far as how soon May discounts are available.....it varies from year to year. Sometimes there are bouncebacks a year in advance, and other times it's just months before the booking dates. You can try checking Mousesavers for historical data for May, some discounts are released pretty much every year and it helps to know when to watch