a vagina contains Skene's glands, Bartholin's glands, and a hymen. Vaginal epithelium is not keratinised, the skin of the penis is. The vagina ends with a cervix unless there is a DSD or the cervix has been surgically removed. A transwoman's surgical construction has none of those things. An inverted glans is not a cervix. An inverted penis or piece of bowel is not a vagina.
No. They are glands etc located near the vagina but are not part of the vagina.
I have never studied keratinisation so I can't comment in detail but my understanding is that only the prepuce is keratinised and, anyway, often penile skin is not used. It is, however, possibly an academic distinction as skin types have a habit of re-mapping themselves following vaginoplasty but the effect has not been studied.