I know there's some controversy on the permissability of women wearing tefillin and that most poskim forbid it. From what I understand that is for women to wear tefillin for the purpose of the mitzvah of tefillin. Is the same true if they are worn as an amulet for protective purposes? Is it still forbidden?
The Mishna Berura (301 sk 158) quotes an argument among Rishonim and Achronim about whether a woman who finds Tefillin in public on Shabbat may wear them as amulets to "carry" them back to a safe place. The debate centers around defining normal modes of wearing clothing. He doesn't cite anyone who suggests that the Rama's exhortation (OC 38:3) of "מוחין בידן" protesting women wearing Tefillin would prevent the woman from "carrying" the Tefillin back to a safe place.
It seems from this that there would be no problem, but perhaps the Mishna Berura's case is unique as the safety of the Tefillin is at stake. On the other hand I don't know of any other case where someone would want to do this.
No it is prohibited for a woman to wear tefillin as an amulet according to Rambam.
It is also forbidden for men to wear tefillin as an amulet.
He writes the following in Hilkhot Avodah Zara (11:13):
יג [יב] הלוחש על המכה וקורא פסוק מן התורה, וכן הקורא על התינוק שלא ייבעת, המניח ספר תורה או תפילין על הקטן בשביל שיישן--לא דיי להן שהן בכלל חוברים ומנחשים: אלא שהן בכלל הכופרים בתורה, שהן עושין דברי תורה רפאות גוף, ואינן אלא רפאות נפשות, שנאמר "ויהיו חיים, לנפשך" (משלי ג,כב).
One who whispers a spell over a wound, at the same time reciting a verse from the Torah, one who recites a verse over a child to save it from terrors, and one who places a Torah scroll or phylacteries on an infant to induce it to sleep, are not merely in the category of sorcerers and soothsayers, but are included among those who repudiate the Torah; for they use its words to cure the body, whereas these are only medicine for the soul, as it is said, "They shall be life to your soul" (Prov. 3:22).