A- Roles of network and PCI SSC

Yes. All business that store, process or transmit payment cardholder data must be PCI Compliant.
Yes. Merely using a third-party company does not exclude a company from PCI compliance. It may cut down on their risk exposure and consequently reduce the effort to validate compliance. However, it does not mean they can ignore PCI.
If your business locations process under the same Tax ID, then typically you are only required to validate once annually for all locations. And, submit quarterly passing network scans by an PCI SSC Approved Scanning Vendor (ASV), if applicable.
In-scope cards include any debit, credit, and pre-paid cards branded with one of the five card association/brand logos that participate in the PCI SSC - American Express, Discover, JCB, MasterCard, and Visa International.
Source : GIE-CB