glass ustensils

Question

Given 1: porcelain dishes glazed with glass do not absorb/retain flavor and therefore do not need kashering. Given 2: glass dishes must be immersed in a mikve for tevila. In both givens, glass is involved, yet the halacha seems to differentiate between these cases. I.E., although glass dishes must have tevila done, glass-glazed porcelain dishes do not require immersion. Q1. Is the reason for this that in Given 1 the glass is "taffel" (considered secondary) to the porcelain [which is exempt from tevila]; whereas in Given 2 the dish is completely glass? On the flipside, glass does not need kashering, yet porcelain-earthenware does. Q2. Regarding kashering, although porcelain / earthenware absorb and needs kashering, if they are glazed with glass they will not absorb/retain flavor -- and therefore the glass is then viewed as the primary part of the dish, and does not require kashering. Correct? Thank you for clarifying.