Since I am planning on staying in Des Moines, is there a mailing address that I could use temporarily for my address?

Yes you can use the CISS address and then come to the shelter and check at front desk for your mail. Mail doesn't come here until around 2pm so you can plan around that time

Yes inside mail boxes

You may want to look into a non-USPS post office box, like those at the UPS store located on Fluer Drive. You get an actual street address.