1. Some ticket printers offer a special feature that allow to operate a cash till: you "print" a special character (or sequence of characters) and, in response, the printer produces an electric signal that can be used (possibly amplified) to command a relay that opens the till (Epson had such models several years ago).
2. Not sure what rules apply where you live. I developed such a system in a Western European country many years ago (1995 for the first version) and encountered very few legal problems. For every of them a solution could be found.
3. Surely yes. The system I developed was for my sister's flowers business. It manages the sales aspects of the business (tickets, legal receipts, fidelity cards), as well as the accounting parts (invoices, taxes, balance sheets, etc.).With several adaptations over the years, it's still in use today.
Be sure to know which legal rules (national and/or local) apply before starting to develop such a project. If necessary, see a lawyer or an accountant first.