Is the proper phrase to console a mourner always the plural המקום ינחם אתכם בתוך שאר אבלי ציון וירושלים or should it be changed depending on whom it is said to (singluar, feminine, etc.)?
In discussing laws associated with consoling mourners, Rabbi Moshe Feinstein (Igrot Moshe OC 5:20:21) uses the phrase in its singular masculine form: המקום ינחם אותך
Rabbi Menashe Klein (Mishneh Halachot 4:144) offers condolences to the recipient of the responsa on the recent loss of his mother, also using the singular masculine form.
I don't remember where I heard this, but someone suggested that just as it is preferred to sit shiva in the house of the niftar (sorry, don't have sources on me), you are giving consolation to both the person/people sitting shiva and the niftar. Thus, even if only one person is siting shiva, there are two people being addressed.
In a case of a woman sitting shiva for a woman, that would still suggest changing אתכם to אתכן, I guess, but I tend to always stick with אתכם.
On a tour in the old city i was informed that the mishna states that mourners would enter the Temple for a separate entrance and walk around the temple in a counterclockwise direction and all others in a clockwise direction. So each oleh who would cross paths with the mourner's line would greet him/her with the plural phrase- referring to the entire line of mourners.
Since mourning is an event during which each mourner is to be reassured that he or she is not entirely deserted and bereft in their loss, the plural is more appropriate as a reminder that even the most personal loss is still borne by a community and that the mourner is still surrounded and supported by that community.