It sounds like PA is one of the worst states for basic human rights, like the freedom to travel. Who knew? That said, I suspect there is
some way to obtain a drivers license. You may have to have it mailed to a friend or family member. But, the right to leave is a basic human right, so, there must be some way to obtain a license without having a home. This homeless law blog discusses some possibilities; it's not specific to PA, but it seems likely there is some mechanism for obtaining a legal license:
http://homelesslaw.wordpress.com/#comment-1496 (I also found a few state and county agencies in PA with mention of drivers licenses and other licenses, though no specific documentation on how to obtain one; I guess calling or emailing would be the way to find out).
I'm not an expert on PA regulations, but I do know that I was able to do everything I needed without a permanent address, including getting a passport, drivers license, car insurance (GEICO offers a special insurance class just for RV full-timers, which is a blend of homeowners and auto insurance), and the postal forms needed for mail forwarding services.
And, one more thing:
"Also, even if I could, somehow, get a mailbox, it would create yet another red flag on my credit reports, because the credit agencies mark, as suspicious, any address which receives mail for many different people."
This is definitely not true. Credit card payments might get flagged if the billing address is a forwarding service (though it has not been the case for me; I've had trouble ordering items shipped to addresses other than my billing address, but I've never had a payment refused because the billing address was a forwarding service), but your credit report will, in no way, be effected by using such a service. I promise you it will not "red flag" your credit reports.
It sounds like you've had a bad run of luck, and it's gotten you feeling like a victim on all fronts. It's understandable. I feel beaten down by bureaucracy sometimes, too. But, you do have some fundamental rights in the US, regardless of whether you have a home or not. Get the ID thing sorted, and everything else will start to fall into place. If PA won't help you get that sorted, raise hell until someone does help you; contact your local state representative or city mayors office or write a letter to the head of the DMV or the local newspaper. You have a right to have an identity. You can't possibly get out of homelessness without one.