I think you must have realized it yourself, namely; what you say applies to the vajrayana visualisations too. While we have our innate buddha-nature to begin with, why should we then practice "artificial" visualisations ?? This really beats me !
Buddha-nature does not fit in Theravāda doctrines, and so Mettā is not innate either. Their practice is to produce such wholesome states through their meditative methods, which is what I am arguing as "artificial". It may lead to personal liberation, but this again is considered "artificial" in Mahāyāna terms.
If your Vajrayāna visualizations are comparable to this (and I know they should not be), then your visualizations are producing your Buddha-nature. In such a case, Buddha-nature would not be innate. Whatever you are developing is therefore not true Buddha-nature.
The original poster said;
"While practicing metta, I see myself in a mirror smiling back at me. As I deepen my concentration I begin to smile at the mirror. It feels good and kind, warm and loving."
This is not practicing true Mettā, nor is it developing it. Perhaps it is in Theravāda, but to the Bodhisattva this is just making yourself feel nice.
In Mahāyāna, Maitrī is a function of Prajñā, which is innate to one's Buddha-nature. Smiling at oneself does not conjure up or foster Prajñā, and therefore not true Maitrī. A better suggestion to that end is to study the Buddha's teachings on the two-fold Śūnyatā.