Nusach Teimanim

Question

1) Would it be permisable for and Ashkenaz to switch his nucash to that of the Teimanim of Sefardim? 2) Would it then be permisable to switch back and forth or would it be nesecary to stick with one nusach? 3) If you do your fathers benching is it ok to switch back and forth between different nuchaot? 4) In the sefardic benching (and tefilos) many words end with "-ach" as opposed to "-echa", which is said by ashkenazim. If an ashkenaz is doing your fathers benching which ending should he use?