Hmm, try this.
Thank you for informing me that the person whose duty it is to pick up the Key has been changed. (If you are the new person, try: Thank you for telling me that it is now my duty to pick up the key.)
I don't know if you are aware that I used to perform the head-count on Monday and Tuesday at 7:00 a.m. Because my schedule has changed, I am no longer available to do this. You may want to give X this responsibility since they are available at those times.
It is OK to use "they" to refer to one person if you do not know whether they are male or female. However, if you know X, you should use "he" or "she":
"You may want to give Jane this responsibility since she is available at those times." OR
"You may want to give John this responsibility since he is available at those times." OR
"You may want to give your assistant the responsibility since they are available at those times." (In this case you don't know the assistant, but you do know their schedule.)