In Teshuvos v'Hanhagos 1:236 (not 136), Rav Sternbuch writes that a Kamei'a that is not Mumcheh may be worn in a Karmelis if there is at least a possibility that it will help the person. The reason is because wearing a Kamei'a outside is only Hotza'ah k'l'Achar Yad , since it is not carried the way a person normally carries out an object (see Mishnah Berurah 308:129).

Regarding your question whether a Kamei'a is a Malbush or Tachshit, it is neither! In the language of the Rishonim, a "Tachshit" is more or less synonomous with a "Malbush" (we find that Rashi uses the term "Tachshit" many times in place of "Malbush").

Perhaps you meant to ask if a Kamei'a is considered a Masuy or Malbush/Tachshit. If so, we find in the Mishnah (60a) that a Kamei'a is in the category of a Masuy, a burden, which is why it is forbidden to go out into Reshus ha'Rabim with a Kamei'a which is not Mumcheh, as we cited from Teshuvos v'Hanhagos above.

Be well!