Yes. Your address belongs to your domain, not the provider. We move the mailbox behind it without changing the address.
No. Migration of existing mail is part of the setup.
Can I just use the free email that came with my domain or hosting? You can, but storage is small, spam filtering is weak, and deliverability is worse. For a business inbox the Google licence pays for itself the first time an important email does not go missing.
One paid mailbox per person. Addresses like info@ or sales@ can usually be free aliases pointing at a real mailbox, and we set those up as part of the job.
The mail itself runs on Google, which is the point: it does not depend on us. If you need changes later, new staff, new aliases, that is a quick job we handle at our standard rates.