A PO box would give you a mailing address, but where is your principal place of business? Sounds like your home--so why would you want to lose out on the deduction? Instead of "renting" space to your S corp, talk to a CPA about taking a deduction as unreimbursed employee business expenses (since, after all, as the sole owner-officer of the S corp, you would be considered an employee of the S corp).

As for the dba, it was issued in your name. The sole proprietorship was never a separate entity--it was you. You can cancel the dba if you're not using it anymore or, if you want your S corp to use the same dba, you can fill out an application on behalf of the S corp for the dba.