1. Magic of Faerun, page 32, (or its freely accessible web excerpt available here) has a sentence that goes as:
If the incantatrix already is a wizard specialized in Abjuration, she does not need to choose another prohibited school.
This sentence seems to have been stated explicitly as an exception, in any other scenario, she must choose additional prohibited schools, increasing the total number of banned schools. (Imagine no such exception had been written. In that scenario, could Abjurers redeclare their already banned schools? If the answer is affirmative, then there would be no meaning to write the exception at all.) Note that Gwendolin can still cast spells from the newly banned schools if they were in her spellbook prior to her becoming an incantatrix.
Let's also have a look at the options available to Gwendolin. According to 3e OGL SRD:
To become an abjurer, a wizard must select a prohibited school or schools from the following choices: (1) either Conjuration, Enchantment, Evocation, Illusion, or Transmutation; or (2) both Divination and Necromancy.
To become a transmuter, a wizard must select a prohibited school or schools from one of the following choices: (1) Conjuration; (2) Evocation; (2) any two of the following three schools: Abjuration, Enchantment, and Illusion; or (4) any three schools.
So, to start with Gwendolin does not need to have Necromancy as a banned school, Evocation is sufficient for her to be a transmuter. Now, to become specialised also in Abjuration, she could choose to sacrifice Divination and Necromancy, for a total of 3 banned schools, or one of Conjuration, Enchantment, or Illusion, for a total of 2 banned schools!
2. She does. Quoting from page 31, she is now:
... a specialist in the school of Abjuration (gaining all the benefits of specializing in a school), ...