Is it permissible to eat before Megillah reading?
The question can be divided into two parts.
Can one eat a small snack, (less than a k’zayit), just to get some sustenance?
Can one actually eat a full meal?
The Shulchan Aruch states (Chapter 672 of Orach Chaim) that someone who is a bit ill and is unable to make it to synagogue to wait until the congregation finishes reading in order to break fast can instead listen to the Megilla reading any time after Plag Hamincha (1¼ halachic hours before sunset). The Rema adds that even if the fast is hard on someone, they still should not eat before Megilla reading. Both of them have their basis in the Terumat Hadeshen.
If one takes a look at the Terumat Hadeshen itself, we see that the basis is Tractate Berachot Daf 4, where it says “One cannot say I will eat some and drink some, and then I will say Shema and pray.” Therefore Tosafot writes on Daf 5 that one cannot eat a meal before saying Shema and praying Arvit.
The Achronim all ask there why should Tosafot write regarding a Torah obligation like Shema. One is only forbidden to eat once the time to recite Shema arrives. When we look a rabbinic obligation like prayer, is one is already forbidden to eat thirty minutes before? R Yechezkel Landau in Tzion Lnefesh Chaya writes that in regards to a rabbinic obligation, if one is already involved in another activity, one does not need to pause the minute the time for the rabbinic obligation arrives, as the Shulchan Aruch writes in Chapter 235. Therefore, the Sages forbid eating a meal 30 minutes before the time of prayer, because were one eating when the time arrived, they would be allowed to continue eating, and they might miss the time to pray altogether. But with regard for the Biblical obligation of Shema, one is required to cease activity when the time arrives, so there was no need to forbid eating a further thirty minutes in advance of dark (1).
However, the Mishna Berura (235:18) writes in the name of Magen Avraham that if they have someone to remind them to pray later, there’s no problem with eating even after the time of Keriat Shema arrives. It appears then, that even for a Biblical obligation, one is allowed to rely on someone to remind them (2). Therefore, the Mishna Berura writes in this case (672:16) that if someone feels weak and is may get sick, and just tasting a little food will not help them feel better, they should be allowed to eat, but only if they have someone to remind them to read the Megilla after they’ve finished eating.
It would seem from the Mishna Berura‘s words, that even though the Megillah reading is only a rabbinic mitzvah, he only allows this solution because of illness. Similarly, when it comes to counting the Omer in Chapter 488, he does not mention anything about having someone to remind you, and it appears that he does not allow this solution. But when he writes about someone waiting on a lulav (652:7), he writes that the Chatam Sofer allows one to eat in the meantime, as the person bringing them the lulav will remind them to shake it once it comes.
It appears that one can can clarify the words of the Mishna Berura in the same manner as the Terumat Hadeshen. Megilla reading is a very strict obligation; we pause other mitzvot in order to read the Megilla. As the Shulchan Aruch writes (687), we even cease Torah learning in order to read Megillah. Similarly, the Eliya Rabba writes that we treat Megillah reading as if it was a Torah obligation. And it seems that we are reluctant to rely on someone to remind one to read the Megillah later. Unlike saying the Shema, Megilla reading is not a daily occurrence, and one is more likely to forget about it.
We find a similar occasion where the lack of regularity makes a difference. On Passover, you cannot eat at the same table as a non-Jew eating bread because you might forget that the bread is forbidden to them. However, you can eat next to a non-Jew who is eating non-kosher meat because there is no way they would get mixed up and eat something that is always forbidden. Similarly, the Gemara writes (Sukkah 26a) that we do not rely on someone to remind you that you need to sleep in the sukkah in order to take a small nap outside it because the person you are relying on will forget and go to sleep outside.
Therefore, in the case of the Megillah reading, which is not a daily occurrence, the Mishna Berura allows relying on someone to remind a person to read, but only in case of illness. But in the case of Shema, where you say it every night, a person is allowed to rely on having someone to remind them.
However, the Kodesh Hazitei of the Shalmat Chaim (Part 1, Chapter 204) explains that the Mishna Berura is actually allowing anyone to rely on someone to remind them, and not just in the case of Keriat Shema. He only mentions getting someone to give a reminder in regard to megilla specifically as advice on what someone feeling weak after their fast should do, because anyone in any circumstance is always able to decide to eat now and rely on having someone to remind them to fulfill their mitzvah after their meal.
Our master, R Ovadia Yosef of blessed memory, in Hazon Ovadia for Purim (p 95) explained the Mishna Berura similarly. So does Birkat Yehuda (Orach Chaim 60) and the Collection of Rulings of R Shmuel Kaminetzky on Purim (74). All these rabbeim would allow one to eat a full meal before the Megillah reading, if that person appointed someone to remind them to read the Megillah. And if someone only wanted a small snack, all these rabbis allow that person to do so — even with no one to remind them to read the Megillah, as many achronim explain the Tosafot that way.
In my opinion, the Mishna Berura was only strict by Sefirat HaOmer because If you miss counting one night, it ruins the counting the rest of the Omer (This is why you cannot count with a blessing going forward), and counting the Omer only occurs at a very specific time of year. However, with regard to what the Hatam Sofer says about Lulav, it doesn’t depend on the person who is waiting to wave the Lulav because they cannot wave the Lulav until the other person comes to bring it to him. And when that happens, it’s certain that the person will wave the Lulav.
If someone is at a synagogue, and there is a long period of time before the reading of the Megillah actually starts, that person could eat less than a k’ziot of mezonot (or a bit more than that of fruit or vegetables and rice). If someone eats more than a k’ziot, they certainly can rely on someone to remind them to read the Megillah, because there are lots of other people in the synagogue who will remind them once the Megillah reading starts. However, if someone is at home and is feeling well, even if they do have someone to remind them, they should be strict and not eat even though it would be allowed by Halacha.
Footnotes
For further info on this debate, see also the Maamar Mordechai, Rosh Yosef, and Olat Tamid on that daf and also the Magen Giborim and R Akiba Eger on the Tosafot there.
Of course, this person needs to be an adult, and not a child. Just like on Shabbat, if one wants to read by the light of one lamp, they can do so, but only if there is an adult reminding them not to tip the lamp to increase the light. We do not trust a child to keep watch.