Starting the Seder after Sheki'ah (sunset)?

Question

In my experience, there is a lot of emphasis on waiting until after Tzeith HaKochavim (the stars come out), which is seen as definitely after nightfall, to start the Seder. The reason is that we want to makes sure that the Mitzvoth of the Seder, and indeed even Kiddush, are done definitely at night. See Mishnah Berurah 472:1:1, for example.

But looking at the Shulhan 'Aruch, and even the Raavad (perhaps the former is based on the latter), it seems clear that there is also an urgency to start the Seder (or at least the meal) early enough that people, especially the children, don't fall asleep (see Mehaber OC 472:1 and see this related question). See also RaAVa"D 7:3 in the RaMBa"M, Laws of leaven and Matzah (from a friend, no time to look up right now).

The word used by the Shu"'A is "משתחשך". The M"B there says, "Lav Davka" ("not quite"). Is the M"B saying, "This doesn't mean quite what you may think it means, so don't mistake the meaning here to think you should start the Seder before nightfall; what is meant here is that you shouldn't wait even later and thereby delay the meal," or is he trying to argue with/alter the Shu"'A, and say, "We don't necessarily have to follow this exactly; the most important thing is to wait until nightfall, and then once we get to nightfall don't delay further"? What I mean is, in a place where nightfall is very late, or even in a situation where the children will definitely either fall asleep or be irritable and the experience will not be a positive one for them, could (or should) a person begin after sunset but before nightfall?

If the first interpretation holds, then it seems, according to M"B, there's no option, and he's clarifying what the Mehaber meant, so that there's no mistake at your or my Seder.

If the second interpretation holds, though, I might think I can start a little earlier if, in my judgment, the quality of the Seder will be affected, since it's important to have the kids awake and participating, and the M"B is just speaking about an ideal situation where everyone's engaged or nobody is young enough to be very much affected by a late start.

Answer 1

I'm going to post how I understand the Mishna Berura's comment, and hopefully that will solve your difficulty.

The Shulchan Aruch states (OC 472:1):

יהיה שלחנו ערוך מבעוד יום, כדי לאכול מיד כשתחשך; ואף אם הוא בבית המדרש, יקום מפני שמצוה למהר ולאכול בשביל התינוקות שלא ישנו, אבל לא יאמר קידוש עד שתחשך.‏
One's table should be set from before the holiday starts in order that one can eat immediately upon darkness. Even if one is in the study hall, one should leave because it is a Mitzva to hurry and eat so the children won't sleep. But he shouldn't say Kiddush before dark.

The Mishna Berura (sk 1) comments on the phrase כדי לאכול מיד "in order that one can eat immediately" saying:

‏(א) כדי לאכול מיד - לאו דוקא והכונה כדי שיהיה אפשר לו להתחיל הסדר תיכף משתחשך ולא ישתהה:‏
In order that one can eat immediately -- Not precisely, and the intention is that they should be able to begin the Seder immediately after dark, and not tarry.

In other words, it seems the Mishna Berura is clarifying that when the Shulchan Aruch said "eat" at nightfall, he meant "begin the Seder" at nightfall, not "eat the Matza" or "eat Shulchan Orech". He still agrees the whole thing must take place after dark. Looking up the Mishna Berura's referenced source (ShA HaRav OC 472:1) one sees this point explicitly:

יהיה שולחנו ערוך מבעוד יום כדי להתחיל הסדר מיד כשתחשך ואף אם הוא בבית המדרש יקום מיד כשתחשך שמצוה למהר להתחיל הסדר בשביל התינוקות שלא ישנו והתורה אמרה והגדת לבנך ביום ההוא: ‏
One's table should be set from before the holiday starts in order that one can begin the seder immediately upon darkness. Even if one is in the study hall...

(translations and emphases mine)

Answer 2

Striking my answer here in light of what I learned from the comments (Thank you @DoubleAA); the answer seems to be more like your first interpretation.

I think the MB is saying neither of those two interpretations but a third, namely Lav Dawka - not specifically - you don't have to rush and time it to eat the matza right at dark, at tzet hakokhavim; if you've made kiddush at sunset or later and you get to matza later than tzet, that's fine by the Mechaber, as long as your start and pace are such that the children won't sleep before eating it. The MB has to say this because he holds stricter as he states in note (Hei) that "aval lo-yomar kidush ad sh'techshakh" is "in other words after emergence of stars" so by the MB there's no way you could start early enough to eat matza at tzet specifically.

Since R' Y. Abadi has written - (and see related question):

מיד כששקעה חמה יכול להתחיל בסדר אך לא יאכל המצות קודם צאת הכוכבים

translation: As soon as the sun sets he can start the seder, but he shouldn't eat the matzot before emergence of stars.

then the according reading of which is more lenient than the SOAC 472:1 which reads:

His table should be set while it's still [pre-sunset] day, in order to eat [matza] as soon as it's gotten dark. So even if he's in the Beit Midrash he should get up because it's a miswah to make haste and eat [i.e start and move along the Seder], for the sake of the children so that they should not sleep. But he should not say Kiddush until it's getting dark [i.e. a sunset]. dark [i.e. emergence of stars]

יהיה שלחנו ערוך מבעוד יום (א)כדי לאכול מיד כשתחשך ואף אם הוא בבית המדרש יקום מפני שמצוה למהר ולאכול בשביל התינוקו' שלא ישנו אבל לא יאמר קידוש (ה)עד שתחשך: MB: (א) כדי לאכול מיד - לאו דוקא והכונה כדי שיהיה אפשר לו להתחיל הסדר תיכף משתחשך ולא ישתהה:‏

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Edit: However I found the following interesting and related, and which may at least support the intentions and desires you mentioned in your second interpretation even if that's not the reading of it. (and which might be the/a basis of RYA's psak above, idk):
A discussion of the Tosefta's language מאימתי אוכלן משתשחך somewhat connoting that its miswah is to eat it מיד כשתשכך and tracing it to R' Eliezer holding (Deut. 16:6) "k'bo ha-shemesh/as (soon as) the sun has set [i.e. dark, tzet hakokhavim]" you eat [the Pesach] - part of the miswah is to eat it then.

ומלשון התוספתא כאן מאימתי אוכלן משתחשך, לא אכלן משתחשך וכו', משמע קצת שמצותו לאכול מיד כשתחשך. וכתב ע"ז באו"ש פ"ו מה' חמץ ומצה סה"ו: וטעמא נראה משום דכתיב כבוא השמש. ועיין לעיל שם שתלאה במחלוקת ר' אליעזר ור' יהושע96 עיי"ש, וסובר ר' אליעזר שכבוא השמש אתה אוכל מצותו בכך. ובתוספות בכורים ציין לבבלי ק"ט א', ועיין טאו"ח ריש סי' תע"ב. ועיין מ"ש להלן פ"ז רה"ב, שו' 6–7, ד"ה ולפיכך. 

Tosefta Kipshuta on Pesachim Perek Bet #67 Hillel haZaken Amud 150
https://www.massorti.com/IMG/pdf/liberman_tosseftamoed4.pdf