Yes, with plugins or a custom automation: sales export, invoices, sync with your software.
Yes: audit of the theme and apps, fixes, then maintenance and further development.
Yes, following PrestaShop standards so that it stays compatible with updates.
Whichever matches your existing systems: Node.js, PHP (Laravel, Symfony), Python or .NET. React only handles the interface.
Yes, often gradually, starting with the pages that need to rank.
For public pages, we use Nuxt, which generates pages Google can read.