Some siddurs have the phrase "Ki Kel Melech Chanun V'Rachum Ata" at the conclusion of Ya'aleh v'Yavo. Some have Melech in parentheses; some don't have it at all. Can anyone tell me something about where these variants come from, and why?
Shulchan Aruch (Orach Chaim 188:3) states that in the third blessing of the Grace After Meals, in which we pray for the restoration of the Davidic kingship, no other kingship - including Hashem's - should be mentioned, "since an earthly kingdom must not be compared to the Heavenly one."
Based on this, Rema there cites Avudraham, who states that "melech" should be omitted in Yaaleh Veyavo, "although I have not seen people customarily doing so."
Taz (subsec. 2) and Magen Avraham (subsec. 2) justify the practice of saying it, on the grounds that the end of Yaaleh Veyavo is pretty far removed from the mention of David's kingdom, so that saying it doesn't evince a lack of respect for Hashem's kingship.
So in short, some versions follow Avudraham (and Rema), others accept Taz' and M.A.'s justification. (Possibly, too, the versions that omit it are influenced by the fact that the phrase "ki Keil... ata" is found in Nehemiah 9:31, without "melech.")
All of this should logically apply only to the Yaaleh Veyavo recited during Birchas Hamazon. I don't know whether there are siddurim that also omit "melech" in the Yaaleh Veyavo recited in the Amidah.