When making a mi-shaberach, without knowing the Hebrew name of the mother, whose name should one use? Can the English name be used?
Yes, the English name should be used. In fact, if the English name is the name by which the mother was generally called during her lifetime, then this is the principle name (your name is what you are called), and it should be used ahead of the Hebrew name (that nobody used).
Sources: Concerning what the ‘principle’ name is, eee Even Ha’ezer 129 (beginning) and Beis Shmuel in the name of Rosh (discussion of called name v. signed name and name for being called up to the Torah).