Questions re: Beginning Issuance from R3

No, that's not really how it works. Any system getting a new certificate needs to also download the intermediate that it was signed with and configure the server to serve it alongside the certificate. Anyone who is trying to validate that a certificate is actually signed by Let's Encrypt needs to check the full certificate chain and validate it against a root certificate, not any specific intermediate certificate. While they did just change the intermediate for the first time in a long while, it could have changed at any point and can change again at any point (such as if the disaster-recovery intermediates start getting used).

If you really need to track the intermediates for some reason (which I guess maybe some DANE implementations do?) then you need to subscribe to the API Announcements to get notified about upcoming new intermediates so you can be proactive about adding them to your system. But there are several that could be used so you'd need to be prepared for certificates that are signed by any of them.