Yes that can be done

15 years would apply for Adult Passports which are valid for 10 years with an expiration period of 5 years. If your previous passport was only valid for 5 years then it is a child Passport, and those cannot be renewed by mail and must be renewed at an acceptance facility or passport agency using form DS-11.

Nope the state department will mail it back using one of their envelopes.

If you are applying from abroad you can use a card.

Money orders can be paid using cash or debit.

You can apply for urgent service and that should get you an appointment with proof of travel. Another option is to do mail in renewal with expedited processing and 1-2 day shipping.

Yes but you would need to apply under a ds-11 with an appointment. If you still have that passport you can use that as your proof of citizenship otherwise use either a CRBA/certificate of citizenship/naturalization certificate or a us birth certificate. Make sure you mark on the ds-11 that you were previously issued a passport. You can also use a file search for $150 if you do not have any of those documents as well as not having your passport though it is cheap and fairly easy to replace a birth certificate.

Yes you can, but you need to log in at around 1 PM EST/10 AM PST as slots are limited. You must be 25 or older and have a had a passport issued after 2009. No name and/or gender changes are allowed with the online renewal system. Only Routine processing is available and pictures must be at 600x600 or higher (1:1 aspect ratio only) with no digital alterations (no keying out the background or use apps that can do that). Photos must be as small as 54 kb to up to 10 MB. This service will revoke your current or expiring passport, so you cannot go around traveling on it.

Use the state departments online appointment system or call to book an appointment a couple of weeks before travel. You must provide proof of travel in order to have an appointment. On the other hand, expedited processing is taking approx 2-3 weeks.

Because you have to mail it to a PO box and only the postal service can be delivered to those. FedEx is a private company and there are no physical addresses on the ds-82 that can be delivered by companies like FedEx. I believe FedEx Ground Economy can deliver to Po box depending on the post office but you are better off getting a priority envelope and mailing it to one of the PO boxes listed on the form.

Awesome 😎