I think your analysis is pretty much correct. From what I understand about RGB is the magic is that its "just bitcoin". So if you don't have a RGB-enabled wallet the transactions are still completely valid, you would only not be able to decipher the RGB-specific metadata attached to transaction.

So, in theory this means that an RGB enabled transaction should operate over LN.

The only thing I don't fully understand, suppose you have a TXN that is 10 sats, but has $10,000 USDT attached. I assume you can (a) Send 1 sat to someone else without touching the USDT (you would just get a change address with 9 sats + 10,000 USDT). (b) Also the reverse, I assume you can send the 10,000 USDT and still retain the 10 sats.....(I'm ignoring mining fees, but you get the idea)