Domain name purchased from Route53 i not needed to be able to get SSL Certificates in ACM. The only resource you need to be able is to have Route53 Zones created, but the domain name can be purchased in any other provider. Of course you need that other provider sets the NS Servers to the ones AWS Route 53 provides
Yes absolutely, feel free to use a domain name purchased from a different provider if you want to. For the lab, it’s just easier to purchase within the AWS console.