1) Yes, it's still possible to convince a phone to connect to older RAT generations. However, the idea is that as those are phased out, it's unlikely that they'll be enabled on UEs, so phones likely won't connect and "say hi" as you say. For example, 2G is already being disabled on many devices. It'll be a while before 2G-4G is fully phased out and IMSI Catchers become completely infeasible, but I think that it's safe to say that "5G got it right" in finally solving this issue.
2) I don't think you have a good understanding of how SUCIs work if you think that it's unique to a device. The UE generates a fresh ECC ephemeral public key every time it sends its SUCI (which isn't often to begin with due to GUTIs, which are one-time use and only assigned post-ciphering). You can read more about it here: https://medium.com/@aditya.koranga/ecies-in-5g-core-supi-to-...
2) I don't think you have a good understanding of how SUCIs work if you think that it's unique to a device. The UE generates a fresh ECC ephemeral public key every time it sends its SUCI (which isn't often to begin with due to GUTIs, which are one-time use and only assigned post-ciphering). You can read more about it here: https://medium.com/@aditya.koranga/ecies-in-5g-core-supi-to-...