2. Question: Why does the Beis HaLevi hold that a person who missed a day of the omer can still say it with a blessing for someone else but someone who is obligated in megilla for Shushan Purim can’t read it for one who needs to hear megilla on regular purim?
A. Rav Tzvi Pesach Frank quoting the Beis HaLevi says that even though you can’t say a blessing anymore by the omer because you skipped a day and there is a doubt whether the mitzvah of omer is one big mitzvah or 49 individual mitzvos so when in doubt you must be lenient and not say a blessing but there is still a concept of “arvus” that we are all connected to each other and help each other fulfill a mitzvah properly if we are both obligated in it therefore he can say a blessing for someone else in order that they can recite the omer with a blessing.
B. The Kaf HaChaim argues and says omer is like reading the megilla just as a person who lives in Yerushalayim and is obligated in shushan purim can’t read megilla for one who is obligated in regular purim even though they are both obligated in megilla, so to one who can’t say the blessing on the omer anymore for himself can’t say it for others.
Answer: By the omer he is really an oness, it is just out of his control that he can’t fulfill the mitzvah if saying the blessing for the omer because of the doubt but he really is obligated possibly but the Yerushalmi has no obligation whatsoever (unless he decides to obligate himself) in the reading for regular Purim. (See Dirshu Mishna Berura 489 footnote 74.)