The Gemara addresses largely in arve pesachim (100 - 106) the problems of havdala at seuda shelishit, the problem of going out without Birkat. In Chulin 86b - 87a the question of the prayer in the middle of a meal if it is an interruption or not.
I want to separate the question in several sub questions.
- One thing is clear for me, this question: When you come back from maariv to make the bentsh at the meal place you need to say retse? In other words it not contradictory to say retse after ata chonantanu and a whole prayer of yom chol? We have a first clear question.
- If we decide to answer to the first question that you cannot say retse, is there a reason to make all your possible to say retse?
- But before we may ask: Is it allowed to go out of the meal place without BHM lechatechilla if you want to continue the meal? The same question may be asked if you clearly decided to finish yet the meal.
- When you go to pray, are there some people who remained around the table and continue the meal?
- Are you obligated to make immediately havdala through Arvit or a cup or both when the night failed?
TO BLESS FOR A MEAL OF SHABBAT ON MOTSAE SHABBAT. The Gemara in Berachot 27b discusses about praying of Shabbat on Erev Shabbat or of motsae Shabbat on Shabbat. In Pesachim 105b, we see that we need to wait for leaving the Shabbat as late as possible, to show that Shabbat is not for us a chore. We can conclude that we need to remain with all the dinim of Shabbat and to say retse even if the time in which we recit the bentsh is already motsae Shabbat. The SA OC 188, 10, says that the reason to say retse is that the meal time is determined by the starting time. See Magen Avraham sk 17, who says that despite that, after maariv he cannot say retse because of the appearance of self contradiction to pray chol and next to bentsh of Shabbat. . The Shulchan Aruch Orach Chayim siman 271, 6 Reports a discussion if the starting time is the criterion or the ending time. A nafka mina is if he already eat before Shabbat, and Shabbat entered before that he had the time to bless Birkat Hamazon (he already made mayim acharonim or said hav lan venivrich). He needs to bench on a first cup and next to make Kiddush on a second cup. Concerning adding retse to the bentsh, the SA reports this doubt. In Bet Yosef there, this question is reported in the name of the Rosh in sugia in Pesachim. Is retse a duty linked to the blessing momentum or to the eating momentum? The Rosh says that e. g. the prayer of tashlumim of Mincha of Shabbat made on Motsash needs to be a prayer of yom chol. The Bet Yosef concluded according to the Orchot Chayim who that the starting time is the criterion. If he wants to continue the meal, this is an other broad issue in Rishonim, not iinked directly to other question.
TO LEAVE THE MEAL EMPLACEMENT BEFORE THE BENTSH. In Gemara Pesachim 101b, there are some examples from which it seems allowed to leave the meal with the intent to come back and eat again. According to the Rashbam there, a special reason is needed to leave before BHM, e. g. to eat in an other place or mehirut Chatan and Kalla.
Authorization to go without bentch there is found only when they want to come back and continuing the meal (Rashi paragraph "keshehen yots'in" p). From the Suggia it seems that if you want not to continue the meal, to leave before Birkat Hamazon is prohibited. Even if you want to continue the meal, you need to make havdala on a cup before continuing (Gemara Pesachim 105a).
If you prayed Maariv without changing place (as you write in comment). The first Tosfot in masechet Chulin 87a reports two opinions concerning Tefila at the middle of a meal. The first, in name of Rabenu Yom Tov, is that blessing or praying makes an interruption, because one can not pray and eat together, the second opinion is that an additional necessary condition is needed to make an interruption the interrupting act should be the end of something. (¹)
Is the premaariv Torah studying a sufficient reason to push the BHM after Arvit? I don't no what to think.
From the example of the Rabbi Yom Tov, we can perhaps see that he was allowing to pray before BHM.
(¹): This part of the question is very well illustrated by the Chiddushe Harashba:
משתא וברוכי בהדי הדדי לא אפשר: יש מרבותינו הצרפתים ז"ל שהיה מוכיח מכאן דפעמים שאדם נזכר בתוך סעודתו שעדיין לא התפלל ומתפלל בתוך סעודתו, טעון ברכה למפרע, ואחר כך נוטל ידיו ובוצע, משום דמיכלי וצלויי בהדי הדדי לא אפשר, וכדאמרינן הכא דמשתי וברוכי בהדי הדדי לא אפשר. וזו דעת רבינו אלפסי ז"ל בפסחים בערבי פסחים גבי ארבע כוסות, והרב בעל התוס' ז"ל כתב דלא דמו כלל, דברכת המזון הוא דחשוב הפסק וכן כיסוי אי לאו טעמא דאפשר למשחט וכסוי בהדי הדדי משום דהני גמר מעשה נינהו, אבל תפילה לא הוי גמר והפסק דאטו מי שברך על הזיקים ועל הזועות בתוך סעודתו או אפילו בורא פרי הגפן מי חשבינן ליה הפסק וחוזר ומברך המוציא משום דמיכל וברוכי בהדי הדדי לא אפשר, ובהדיא אמרינן התם בפרק ערבי פסחים (קב, א) עקרו רגליהם לילך לבית הכנסת והניחו שם מקצת חברי' כשהן חוזרין אינן טעונין ברכה למפרע ולא לכתחילה, ואף על פי שמסתמא התפללו בנתים, וזו היא דעת רבי זרחיה ז"ל שם בפרק ערבי פסחים, וזה נ"ל עיקר, ויש עוד ראיות אחרות.
There is a French Rabbi who was giving a proof from the Gemara in Chulin which says that blessing is an interruption regarding meals, because blessing and eating together is impossible. The rule is the following: If someone was eating and suddenly recalls that he yet did not praying, and he prays at the middle of the meal, after the Tefila he have to bless beracha acharona {in Tosfot too, B.H.M. is explicitly addressed but not in Rosh there, and in Tur OC 178. This is linked to an other Machloket between Rif and Baal Hamaor regarding two motsi for one B.H.M.} and next to wash the hands and bless hamotsi a second time on the bread.
The RMA OC 178, 2 said that for a Mitsva it is allowed to leave without BHM, e. g. to go to pray. See Beur Halacha there and see Bet Yosef which compares the din of Tefila to the lecture of the Haggada between first and second cup of lel haseder. .
--> To bentsh before praying maariv without shinui makom. From the Rama if it is a mitsva overet, e. g. the only minyan is here and now, - - > Not necessarily,
mitsva lo overet - - >Yes, this is an obligation.