This might be a stupid question, but I want to confirm וּמֵיהוה is pronounced u-may-adoni. The reason I ask is לַייָ is kind of blended to be la-donoi (the English letters might be a bit off but hopefully people understand what I mean).
You are correct that when there is a "מ" before Hashem's name the word should be pronounced "u-may-adoni". Only a Mem - Shin - and Hey that are before Hashem's name [מש"ה סימן] have the Alef vocalized. On one of the first pages of an Artscroll siddur they print a box with Hashem's name and how it should be vocalized with different prefixes.